diff --git a/collects/mzlib/kw.ss b/collects/mzlib/kw.ss
index def1f7e..81b2e29 100644
--- a/collects/mzlib/kw.ss
+++ b/collects/mzlib/kw.ss
@@ -514,8 +514,7 @@
 ;; raise an appropriate exception
 (define (error* who fmt . args)
   (raise (make-exn:fail:contract
-          (string->immutable-string
-           (apply format (string-append "~a: " fmt) who args))
+          (apply format (string-append "~a: " fmt) who args)
           (current-continuation-marks))))
 
 ;; keyword searching utility (note: no errors for odd length)
diff --git a/collects/mzlib/list.ss b/collects/mzlib/list.ss
index f4bd0ce..fbad313 100644
--- a/collects/mzlib/list.ss
+++ b/collects/mzlib/list.ss
@@ -238,10 +238,8 @@
               [(not (pair? l))
                (raise
                 (make-exn:fail:contract
-                 (string->immutable-string
-                  (format
-                   "~a: second argument must be a (proper) list; given ~e"
-                   'name list))
+                 (format "~a: second argument must be a (proper) list; given ~e"
+                         'name list)
                  (current-continuation-marks)))]
               [else (let ([a (car l)])
                       #,(case (syntax-e #'mode)
diff --git a/collects/mzlib/private/sigmatch.ss b/collects/mzlib/private/sigmatch.ss
index 784a535..84d0fc4 100644
--- a/collects/mzlib/private/sigmatch.ss
+++ b/collects/mzlib/private/sigmatch.ss
@@ -46,13 +46,12 @@
 		(when (eq? v no-val)
 		  (raise
 		   (make-exn:fail:unit
-		    (string->immutable-string
-		     (format
-		      "~a: ~a is missing a value name `~a', required by ~a"
-		      who
-		      src-context
-		      (sig-path-name s path)
-		      dest-context))
+		    (format
+                     "~a: ~a is missing a value name `~a', required by ~a"
+                     who
+                     src-context
+                     (sig-path-name s path)
+                     dest-context)
 		    (current-continuation-marks))))
 		(and v
 		     (begin
@@ -60,14 +59,13 @@
 			 (let ([p (sig-path-name s path)])
 			   (raise
 			    (make-exn:fail:unit
-			     (string->immutable-string
-			      (format
-			       "~a: ~a contains `~a' as a sub-unit name, but ~a contains `~a' as a value name"
-			       who
-			       src-context
-			       p
-			       dest-context
-			       p))
+			     (format
+                              "~a: ~a contains `~a' as a sub-unit name, but ~a contains `~a' as a value name"
+                              who
+                              src-context
+                              p
+                              dest-context
+                              p)
 			     (current-continuation-marks)))))
 		       (hash-table-put! table s #f)
 		       #t)))]
@@ -76,13 +74,12 @@
 		(when (eq? v no-val)
 		  (raise
 		   (make-exn:fail:unit
-		    (string->immutable-string
-		     (format
-		      "~a: ~a is missing a sub-unit name `~a', required by ~a"
-		      who
-		      src-context
-		      (sig-path-name (car s) path)
-		      dest-context))
+		    (format
+                     "~a: ~a is missing a sub-unit name `~a', required by ~a"
+                     who
+                     src-context
+                     (sig-path-name (car s) path)
+                     dest-context)
 		    (current-continuation-marks))))
 		(and v
 		     (begin
@@ -90,14 +87,13 @@
 			 (let ([p (sig-path-name (car s) path)])
 			   (raise
 			    (make-exn:fail:unit
-			     (string->immutable-string
-			      (format
-			       "~a: ~a contains `~a' as a value name, but ~a contains `~a' as a sub-unit name"
-			       who
-			       src-context
-			       p
-			       dest-context
-			       p))
+			     (format
+                              "~a: ~a contains `~a' as a value name, but ~a contains `~a' as a sub-unit name"
+                              who
+                              src-context
+                              p
+                              dest-context
+                              p)
 			     (current-continuation-marks)))))
 		       (hash-table-put! table (car s) #f)
 		       (check-sig-match v (cdr s) (cons (car s) path)
@@ -113,14 +109,13 @@
 		  (let ([p (sig-path-name k path)])
 		    (raise
 		     (make-exn:fail:unit
-		      (string->immutable-string
-		       (format
-			"~a: ~a contains an extra ~a name `~a' that is not required by ~a"
-			who
-			src-context
-			(if (symbol? v) 'value 'sub-unit)
-			p
-			dest-context))
+		      (format
+                       "~a: ~a contains an extra ~a name `~a' that is not required by ~a"
+                       who
+                       src-context
+                       (if (symbol? v) 'value 'sub-unit)
+                       p
+                       dest-context)
 		      (current-continuation-marks)))))))
 	     #t)))
 
diff --git a/collects/mzlib/thread.ss b/collects/mzlib/thread.ss
index fa1ad7c..8f717fb 100644
--- a/collects/mzlib/thread.ss
+++ b/collects/mzlib/thread.ss
@@ -46,9 +46,8 @@
            (unless (procedure-arity-includes? f num)
              (raise
               (make-exn:fail:contract:arity
-               (string->immutable-string
-                (format "<procedure-from-consumer-thread>: consumer procedure arity is ~e; provided ~s argument~a"
-                        (procedure-arity f) num (if (= 1 num) "" "s")))
+               (format "<procedure-from-consumer-thread>: consumer procedure arity is ~e; provided ~s argument~a"
+                       (procedure-arity f) num (if (= 1 num) "" "s"))
                (current-continuation-marks)))))
          (semaphore-wait protect)
          (set! front-state (cons new-state front-state))
diff --git a/collects/mzlib/unit200.ss b/collects/mzlib/unit200.ss
index 0a47334..0479eb7 100644
--- a/collects/mzlib/unit200.ss
+++ b/collects/mzlib/unit200.ss
@@ -371,17 +371,15 @@
     (unless (unit? unit)
       (raise
        (make-exn:fail:unit
-	(string->immutable-string
-	 (format "compound-unit: result of expression for tag ~s not a unit: ~e" tag unit))
+	(format "compound-unit: result of expression for tag ~s not a unit: ~e" tag unit)
 	(current-continuation-marks))))
     (unless (= num-imports (unit-num-imports unit))
       (raise
        (make-exn:fail:unit
-	(string->immutable-string
-	 (format "compound-unit: unit for tag ~s expects ~a imports, given ~a" 
-		 tag
-		 (unit-num-imports unit)
-		 num-imports))
+	(format "compound-unit: unit for tag ~s expects ~a imports, given ~a" 
+                tag
+                (unit-num-imports unit)
+                num-imports)
 	(current-continuation-marks))))
     (list->vector
      (map (lambda (ex)
@@ -390,9 +388,8 @@
 	       [(null? l)
 		(raise
 		 (make-exn:fail:unit
-		  (string->immutable-string
-		   (format "compound-unit: unit for tag ~s has no ~s export" 
-			   tag ex))
+		  (format "compound-unit: unit for tag ~s has no ~s export" 
+                          tag ex)
 		  (current-continuation-marks)))]
 	       [(eq? (car l) ex)
 		i]
@@ -757,15 +754,13 @@
     (unless (unit? u)
       (raise
        (make-exn:fail:unit
-	(string->immutable-string
-	 (format "invoke-unit: result of unit expression was not a unit: ~e" u))
+	(format "invoke-unit: result of unit expression was not a unit: ~e" u)
 	(current-continuation-marks))))
     (unless (= (unit-num-imports u) n)
       (raise
        (make-exn:fail:unit
-	(string->immutable-string
-	 (format "invoke-unit: expected a unit with ~a imports, given one with ~a imports"
-		 n (unit-num-imports u)))
+	(format "invoke-unit: expected a unit with ~a imports, given one with ~a imports"
+                n (unit-num-imports u))
 	(current-continuation-marks)))))
 
   ;; ----------------------------------------------------------------------
diff --git a/collects/mzlib/unitsig200.ss b/collects/mzlib/unitsig200.ss
index 551b0d1..f0a744b 100644
--- a/collects/mzlib/unitsig200.ss
+++ b/collects/mzlib/unitsig200.ss
@@ -181,10 +181,8 @@
 	   (unless (signed-unit? u)
 	     (raise
 	      (make-exn
-	       (string->immutable-string
-		(format
-		 "~s: expression for \"~s\" is not a signed unit: ~e"
-		 who tag u))
+	       (format "~s: expression for \"~s\" is not a signed unit: ~e"
+                       who tag u)
 	       (current-continuation-marks)))))
 	 units tags)
 	(for-each
@@ -204,10 +202,9 @@
 	     (unless (= c n)
 	       (raise
 		(make-exn
-		 (string->immutable-string
-		  (format
-		   "~s: ~a unit imports ~a units, but ~a units were provided"
-		   who tag n c))
+		 (format
+                  "~s: ~a unit imports ~a units, but ~a units were provided"
+                  who tag n c)
 		 (current-continuation-marks))))))
 	 units tags isigs)
 	(for-each
diff --git a/collects/net/cookie-unit.ss b/collects/net/cookie-unit.ss
index 8b5317e..dd2d5ac 100644
--- a/collects/net/cookie-unit.ss
+++ b/collects/net/cookie-unit.ss
@@ -64,10 +64,8 @@
   ;; constructs a cookie-error struct from the given error message
   ;; (added to fix exceptions-must-take-immutable-strings bug)
   (define (error* fmt . args)
-    (raise
-     (make-cookie-error
-       (string->immutable-string (apply format fmt args))
-       (current-continuation-marks))))
+    (raise (make-cookie-error (apply format fmt args)
+                              (current-continuation-marks))))
 
   ;; The syntax for the Set-Cookie response header is
   ;; set-cookie      =       "Set-Cookie:" cookies
diff --git a/collects/net/nntp-unit.ss b/collects/net/nntp-unit.ss
index aac80a1..e07fa81 100644
--- a/collects/net/nntp-unit.ss
+++ b/collects/net/nntp-unit.ss
@@ -38,7 +38,7 @@
   (define (signal-error constructor format-string . args)
     (lambda exn-args
       (raise (apply constructor
-                    (string->immutable-string (apply format format-string args))
+                    (apply format format-string args)
                     (current-continuation-marks)
                     exn-args))))
 
diff --git a/collects/net/pop3-unit.ss b/collects/net/pop3-unit.ss
index 2e9ce64..d0b754c 100644
--- a/collects/net/pop3-unit.ss
+++ b/collects/net/pop3-unit.ss
@@ -32,8 +32,7 @@
   (define (signal-error constructor format-string . args)
     (lambda exn-args
       (raise (apply constructor
-                    (string->immutable-string
-                     (apply format format-string args))
+                    (apply format format-string args)
                     (current-continuation-marks)
                     exn-args))))
 
diff --git a/collects/net/url-unit.ss b/collects/net/url-unit.ss
index 8b3fc4d..34603a0 100644
--- a/collects/net/url-unit.ss
+++ b/collects/net/url-unit.ss
@@ -60,12 +60,11 @@
                 v)))))
 
     (define (url-error fmt . args)
-      (let ([s (string->immutable-string
-                (apply format fmt
-                       (map (lambda (arg)
-                              (if (url? arg) (url->string arg) arg))
-                            args)))])
-        (raise (make-url-exception s (current-continuation-marks)))))
+      (raise (make-url-exception
+              (apply format fmt
+                     (map (lambda (arg) (if (url? arg) (url->string arg) arg))
+                          args))
+              (current-continuation-marks))))
 
     (define (url->string url)
       (let ([scheme (url-scheme url)]