diff --git a/collects/r6rs/private/encode-name.ss b/collects/r6rs/private/encode-name.ss
new file mode 100644
index 0000000000..71f36d210e
--- /dev/null
+++ b/collects/r6rs/private/encode-name.ss
@@ -0,0 +1,24 @@
+#lang scheme/base
+
+(provide encode-name)
+
+(define (encode-name s)
+  (let ([s (if (symbol? s)
+               (symbol->string s)
+               s)])
+    (cond
+     [(regexp-match #rx"(.*?)([^a-zA-Z0-9_+-]+)(.*)" s)
+      => (lambda (m)
+           (string-append
+            (cadr m)
+            (apply
+             string-append
+             (map (lambda (c)
+                    (let ([s (format "0~x" c)])
+                      (string-append
+                       "%"
+                       (substring s (- (string-length s) 2)))))
+                  (bytes->list (string->bytes/utf-8 (caddr m)))))
+            (encode-name (cadddr m))))]
+     [else s])))
+
diff --git a/collects/r6rs/private/parse-ref.ss b/collects/r6rs/private/parse-ref.ss
index 271881e244..47841d4a86 100644
--- a/collects/r6rs/private/parse-ref.ss
+++ b/collects/r6rs/private/parse-ref.ss
@@ -1,6 +1,7 @@
 #lang scheme/base
 
 (require "find-version.ss"
+         "encode-name.ss"
          (for-template scheme/base))
 
 (provide parse-import)
@@ -37,10 +38,19 @@
           (andmap identifier? (syntax->list #'(id2 ...)))
           (is-version-reference? #'(vers ...)))
      (let-values ([(coll file)
-                   (let ([strs (map (lambda (id)
-                                      (symbol->string (syntax-e id)))
-                                    (syntax->list #'(id1 id2 ...)))])
-                     (if (= 1 (length strs))
+                   (let* ([strs (map (lambda (id)
+                                       (symbol->string (syntax-e id)))
+                                     (syntax->list #'(id1 id2 ...)))]
+                          [len (length strs)]
+                          [strs (map
+                                 encode-name
+                                 (if (and (= 2 len) (regexp-match? #rx"^main_*$" (cadr strs)))
+                                     ;; rename (X main_*) => (X main__*)
+                                     (list (car strs)
+                                           (string-append (cadr strs) "_"))
+                                     ;; no rename
+                                     strs))])
+                     (if (= 1 len) 
                          (values (list (car strs)) "main")
                          (values (reverse (cdr (reverse strs)))
                                  (car (reverse strs)))))])
diff --git a/collects/r6rs/run.ss b/collects/r6rs/run.ss
index 2c912ad9fa..6db517a710 100644
--- a/collects/r6rs/run.ss
+++ b/collects/r6rs/run.ss
@@ -6,7 +6,8 @@
          setup/dirs
          scheme/port
          scheme/file
-         "private/readtable.ss")
+         "private/readtable.ss"
+         "private/encode-name.ss")
 
 (define install-mode (make-parameter #f))
 (define compile-mode (make-parameter #f))
@@ -123,11 +124,23 @@
                      (loop (cdr name)))]))))
 
 (define (name->path name)
-  (let* ([name (if (or (= (length name) 1)
-                       (and (= (length name) 2)
+  (let* ([name (let ([len (length name)])
+                 (cond
+                  [(or (= len 1)
+                       (and (= len 2)
                             (not (symbol? (cadr name)))))
-                   (list* (car name) 'main (cdr name))
-                   name)])
+                   ;; Add implicit "main":
+                   (list* (car name) 'main (cdr name))]
+                  [(and (or (= len 2)
+                            (and (= len 3)
+                                 (not (symbol? (caddr name)))))
+                        (regexp-match #rx"^main_*$" (symbol->string (cadr name))))
+                   ;; Rename (X main_*) => (X main__*)
+                   (list* (car name)
+                          (string->symbol
+                           (string-append (symbol->string (cadr name)) "_"))
+                          (cddr name))]
+                  [else name]))])
     (apply build-path
            (if (install-all-users)
                (find-collects-dir)
@@ -140,7 +153,7 @@
                ;; versioned:
                (list
                 (format "~a~a.ss"
-                        (car name)
+                        (encode-name (car name))
                         (apply
                          string-append
                          (map (lambda (v)
@@ -148,9 +161,9 @@
                               (cadr name)))))]
               [(null? (cdr name))
                ;; unversioned:
-               (list (format "~a.ss" (car name)))]
+               (list (format "~a.ss" (encode-name (car name))))]
               [else
-               (cons (symbol->string (car name))
+               (cons (encode-name (car name))
                      (loop (cdr name)))])))))
 
 ;; ----------------------------------------
diff --git a/collects/r6rs/scribblings/r6rs.scrbl b/collects/r6rs/scribblings/r6rs.scrbl
index 695103be31..0ee8b4e2af 100644
--- a/collects/r6rs/scribblings/r6rs.scrbl
+++ b/collects/r6rs/scribblings/r6rs.scrbl
@@ -175,8 +175,9 @@ a name is converted to a PLT Scheme module pathname (see @secref[#:doc
 guide-src "module-paths"]) by concatenating the symbols with a
 @litchar{/} separator, and then appending the version integers each
 with a preceeding @litchar{-}. As a special case, when an @|r6rs| path
-contains a single symbol followed by a version, a @schemeidfont{main}
-symbol is effectively inserted after the initial symbol.
+contains a single symbol (optionally followed by a version), a
+@schemeidfont{main} symbol is effectively inserted after the initial
+symbol. See below for further encoding considerations.
 
 When an @|r6rs| library or top-level program refers to another
 library, it can supply version constraints rather than naming a
@@ -190,13 +191,41 @@ search order for file extensions is @filepath{.mzscheme.ss},
 resolving version constraints, these extensions are all tried when
 looking for matches.
 
+
+
+To ensure that all @|r6rs| library names can be converted to a unique
+and distinct library module path, the following conversions are
+applied to each symbol before concatenating them:
+
+@itemize{
+
+ @item{The symbol is encoded using UTF-8, and the resulting bytes are
+ treated as Latin-1 encoded characters. ASCII letters, digits,
+ @litchar{+}, @litchar{-}, and @litchar{_} are left as-is; other
+ characters are replaced by @litchar{%} followed by two lowercase
+ hexadecimal digits. Note that UTF-8 encodes ASCII letters, digits,
+ @|etc| as themselves, so typical library names correspond to readable
+ module paths.}
+
+ @item{If the @|r6rs| library reference has two symbol elements and
+ the second one is @schemeidfont{main} followed by any number of
+ underscores, then an extra underscore is added to that symbol. This
+ conversion avoids a collision between an explicit @schemeidfont{main}
+ and the implicit @schemeidfont{main} when a library path has a single
+ symbol element.}
+
+}
+
 Examples (assuming a typical PLT Scheme installation):
 
 @schemeblock[
 (rnrs io simple (6))  #, @elem{means}  (lib "rnrs/io/simple-6.ss")
 (rnrs)                #, @elem{means}  (lib "rnrs/main-6.ss")
+(rnrs main)           #, @elem{means}  (lib "rnrs/main_.ss")
 (rnrs (6))            #, @elem{means}  (lib "rnrs/main-6.ss")
 (scheme base)         #, @elem{means}  (lib "scheme/base.ss")
+(achtung!)            #, @elem{means}  (lib "achtung%21/main.ss")
+(funco new-λ)         #, @elem{means}  (lib "funco/new-%ce%bb.ss")
 ]
 
 
diff --git a/collects/scribblings/reference/syntax.scrbl b/collects/scribblings/reference/syntax.scrbl
index d4de9a45eb..f5849efa9e 100644
--- a/collects/scribblings/reference/syntax.scrbl
+++ b/collects/scribblings/reference/syntax.scrbl
@@ -325,10 +325,20 @@ corresponds to the default @tech{module name resolver}.
  @litchar{/} is the path delimiter (multiple adjacent @litchar{/}s are
  treated as a single delimiter), @litchar{..} accesses the parent
  directory, and @litchar{.} accesses the current directory. The path
- cannot be empty or contain a leading or trailing slash, path
- elements before than the last one cannot include a file suffix,
+ cannot be empty or contain a leading or trailing slash, path elements
+ before than the last one cannot include a file suffix (i.e., a
+ @litchar{.} in an element other than @litchar{.} or @litchar{..}),
  and the only allowed characters are ASCII letters, ASCII digits,
- @litchar{-}, @litchar{+}, @litchar{_}, @litchar{.}, and @litchar{/}.}
+ @litchar{-}, @litchar{+}, @litchar{_}, @litchar{.}, @litchar{/}, and
+ @litchar{%}. Furthermore, a @litchar{%} is allowed only when followed
+ by two lowercase hexadecimal digits, and the digits must form a
+ number that is not the ASCII value of a letter, digit, @litchar{-},
+ @litchar{+}, or @litchar{_}.
+
+ @margin-note{The @litchar{%} provision is intended to support a
+ one-to-one encoding of arbitrary strings as path elements (after
+ UTF-8 encoding). Such encodings are not decoded to arrive at a
+ filename, but instead preserved in the file access.}}
 
  @defsubform[(lib rel-string ...+)]{A path to a module installed into
  a @tech{collection} (see @secref["collects"]). The @scheme[rel-string]s in
@@ -406,17 +416,19 @@ corresponds to the default @tech{module name resolver}.
  ]
 
  and where an @nonterm{elem} is a non-empty sequence of characters
- that are ASCII letters, ASCII digits, @litchar{-}, @litchar{+}, or
- @litchar{_}, and an @nonterm{int} is a non-empty sequence of ASCII
- digits. As this shorthand is expended, a @filepath{.plt} extension is
- added to @nonterm{pkg}, and a @filepath{.ss} extension is added to
+ that are ASCII letters, ASCII digits, @litchar{-}, @litchar{+},
+ @litchar{_}, or @litchar{%} followed by lowercase hexadecimal digits
+ (that do not encode one of the other allowed characters), and an
+ @nonterm{int} is a non-empty sequence of ASCII digits. As this
+ shorthand is expended, a @filepath{.plt} extension is added to
+ @nonterm{pkg}, and a @filepath{.ss} extension is added to
  @scheme{path}; if no @nonterm{path} is included, @filepath{main.ss}
  is used in the expansion.
 
  A @scheme[(planet string)] form is like a @scheme[(planet id)] form
  with the identifier converted to a string, except that the
- @scheme[string] can optionally end with a file extension for a
- @nonterm{path}.
+ @scheme[string] can optionally end with a file extension (i.e., a
+ @litchar{.}) for a @nonterm{path}.
 
  In the more general last form of a @scheme[planet] module path, the
  @scheme[rel-string]s are similar to the @scheme[lib] form, except
diff --git a/collects/tests/mzscheme/module.ss b/collects/tests/mzscheme/module.ss
index 968061ad56..5223abf88d 100644
--- a/collects/tests/mzscheme/module.ss
+++ b/collects/tests/mzscheme/module.ss
@@ -307,14 +307,24 @@
 (test #t module-path? "hello")
 (test #t module-path? "hello.ss")
 (test #f module-path? "hello*ss")
+(test #t module-path? "hello%2ess")
+(test #t module-path? "hello%00ss")
+(test #f module-path? "hello%2Ess")
+(test #f module-path? "hello%41ss")
+(test #f module-path? "hello%4")
+(test #f module-path? "hello%")
+(test #f module-path? "hello%q0")
+(test #f module-path? "hello%0q")
 (test #f module-path? "foo.ss/hello")
 (test #f module-path? "foo/")
 (test #f module-path? "a/foo/")
 (test #f module-path? "/foo.ss")
 (test #f module-path? "/a/foo.ss")
 (test #f module-path? "a/foo.ss/b")
+(test #t module-path? "a/foo%2ess/b")
 (test #t module-path? "a/_/b")
 (test #t module-path? "a/0123456789+-_/b.---")
+(test #t module-path? "a/0123456789+-_/b.-%2e")
 (test #t module-path? "../foo.ss")
 (test #t module-path? "x/../foo.ss")
 (test #t module-path? "x/./foo.ss")
@@ -324,6 +334,8 @@
 (test #t module-path? 'hello)
 (test #f module-path? 'hello/)
 (test #f module-path? 'hello.ss)
+(test #t module-path? 'hello%2ess)
+(test #f module-path? 'hello%2Ess)
 (test #f module-path? 'hello/a.ss)
 (test #f module-path? '/hello/a.ss)
 (test #f module-path? '/hello)
@@ -356,6 +368,9 @@
 (test #f module-path? '(planet))
 (test #f module-path? '(planet robby))
 (test #t module-path? '(planet robby/redex))
+(test #t module-path? '(planet robby%2e/%2eredex))
+(test #f module-path? '(planet robby%2/redex))
+(test #f module-path? '(planet robby/redex%2))
 (test #f module-path? '(planet robby/redex/))
 (test #f module-path? '(planet robby/redex/foo/))
 (test #f module-path? '(planet /robby/redex/foo))
@@ -373,6 +388,7 @@
 (test #t module-path? '(planet robby/redex:7:8-9/foo))
 (test #t module-path? '(planet robby/redex:7:8-9))
 (test #t module-path? '(planet robby/redex:700:800-00900/foo))
+(test #t module-path? '(planet robby/redex:700:800-00900/foo%2e))
 (test #f module-path? '(planet robby/redex:=7/foo))
 (test #f module-path? '(planet robby/redex::8/foo))
 (test #f module-path? '(planet robby/redex:7:/foo))
@@ -389,6 +405,7 @@
 (test #t module-path? '(planet "foo.ss" ("robby" "redex.plt" 7 8)))
 (test #t module-path? '(planet "foo.ss" ("robby" "redex.plt" 7 (= 8))))
 (test #t module-path? '(planet "foo.ss" ("robby" "redex.plt") "sub" "deeper"))
+(test #t module-path? '(planet "foo%2e.ss" ("robby%2e" "redex%2e.plt") "sub%2e" "%2edeeper"))
 
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
diff --git a/src/mzscheme/src/module.c b/src/mzscheme/src/module.c
index fda8a8f2e2..ac04da65b9 100644
--- a/src/mzscheme/src/module.c
+++ b/src/mzscheme/src/module.c
@@ -1818,6 +1818,39 @@ static Scheme_Object *namespace_unprotect_module(int argc, Scheme_Object *argv[]
   return scheme_void;
 }
 
+static int plain_char(int c)
+{
+  return (((c >= 'a') && (c <= 'z'))
+          || ((c >= 'A') && (c <= 'Z'))
+          || ((c >= '0') && (c <= '9'))
+          || (c == '-')
+          || (c == '_')
+          || (c == '+'));
+}
+
+static int ok_hex(int c)
+{
+  return (((c >= 'a') && (c <= 'f'))
+          || ((c >= '0') && (c <= '9')));
+}
+
+static int ok_escape(int c1, int c2)
+{
+  c1 = (((c1 >= 'a') && (c1 <= 'f'))
+        ? (c1 - 'a' + 10)
+        : (c1 - '0'));
+  c2 = (((c2 >= 'a') && (c2 <= 'f'))
+        ? (c2 - 'a' + 10)
+        : (c2 - '0'));
+
+  c1 = (c1 << 4) + c2;
+  
+  if (plain_char(c1))
+    return 0;
+  else
+    return 1;
+}
+
 static int ok_path_string(Scheme_Object *obj, int dir_ok, int just_file_ok, int file_end_ok, int for_planet)
 {
   mzchar *s = SCHEME_CHAR_STR_VAL(obj);
@@ -1929,12 +1962,11 @@ static int ok_path_string(Scheme_Object *obj, int dir_ok, int just_file_ok, int
       }
       prev_was_slash = 0;
     } else {
-      if (((c >= 'a') && (c <= 'z'))
-          || ((c >= 'A') && (c <= 'Z'))
-          || ((c >= '0') && (c <= '9'))
-          || (c == '-')
-          || (c == '_')
-          || (c == '+')) {
+      if (plain_char(c)
+          || ((c == '%')
+              && ok_hex(s[i+1])
+              && ok_hex(s[i+2])
+              && ok_escape(s[i+1], s[i+2]))) {
         prev_was_slash = 0;
       } else if ((i < start_package_pos) || (i >= end_package_pos))
         return 0;
@@ -1998,13 +2030,12 @@ static int ok_planet_string(Scheme_Object *obj)
 
   while (i--) {
     c = s[i];
-    if (((c >= 'a') && (c <= 'z'))
-        || ((c >= 'A') && (c <= 'Z'))
-        || ((c >= '0') && (c <= '9'))
-        || (c == '.')
-        || (c == '-')
-        || (c == '_')
-        || (c == '+')) {
+    if ((c == '%')
+        && ok_hex(s[i+1])
+        && ok_hex(s[i+2])
+        && ok_escape(s[i+1], s[i+2])) {
+      /* ok */
+    } else if (plain_char(c) || (c == '.')) {
       /* ok */
     } else
       return 0;