diff --git a/collects/compiler/embed-unit.rkt b/collects/compiler/embed-unit.rkt
index dabab2df69..bf635cef84 100644
--- a/collects/compiler/embed-unit.rkt
+++ b/collects/compiler/embed-unit.rkt
@@ -175,7 +175,14 @@
                              creator
                              
                              "CFBundleIdentifier" 
-                             (format "org.racket-lang.~a" (path->string name)))]
+                             (format "org.racket-lang.~a~a" 
+                                     (path->string name)
+                                     (if (fixnum? (expt 2 32))
+                                         ;; Add a "-64" suffix for 64-bit, because Lion seems
+                                         ;; to get confused by 32-bit and 64-bit apps with the same
+                                         ;; id (see PR 12135)
+                                         "-64"
+                                         "")))]
                  [new-plist (if uti-exports
                                 (plist-replace
                                  new-plist
diff --git a/src/mac/osx_appl.rkt b/src/mac/osx_appl.rkt
index b2d2e26520..c5ad175439 100644
--- a/src/mac/osx_appl.rkt
+++ b/src/mac/osx_appl.rkt
@@ -86,7 +86,11 @@
 	   (assoc-pair "CFBundleExecutable"
 		       ,app-name)
 	   (assoc-pair "CFBundleIdentifier"
-		       ,(format "org.racket-lang.~a" app-name))
+		       ,(format "org.racket-lang.~a~a" app-name
+                                (if (fixnum? (expt 2 32))
+                                    ;; See comment on similar code in compiler/embed-unit:
+                                    "-64"
+                                    "")))
 	   ,@(if app?
 		 `((assoc-pair "CFBundleIconFile"
 			       ,app-name))