diff --git a/ChangeLog b/ChangeLog
index 751fbfd32..13047d535 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2015-09-04 Niels Grewe <niels.grewe@halbordnung.de>
+
+	* Source/GSString.m: Optimisation for formatting strings that are stored as
+	UTF-16 (use memcpy instead of looping over the characters).
+	* Tests/base/NSString/unichar_format.m: Test cases for formatting.
+
 2015-09-01 Niels Grewe <niels.grewe@halbordnung.de>
 
 	* Source/NSRegularExpression.m: Fix handling of empty capture groups.
diff --git a/Source/GSString.m b/Source/GSString.m
index 379b2f9c8..30bec69de 100644
--- a/Source/GSString.m
+++ b/Source/GSString.m
@@ -6267,12 +6267,8 @@ GSPrivateStrAppendUnichars(GSStr s, const unichar *u, unsigned l)
    */
   if (s->_flags.wide == 1)
     {
-      unsigned 	i;
-
-      for (i = 0; i < l; i++)
-	{
-	  s->_contents.u[s->_count++] = u[i];
-	}
+      memcpy(s->_contents.u + s->_count, u, l * sizeof(unichar));
+      s->_count += l;
     }
   else
     {
diff --git a/Tests/base/NSString/unichar_format.m b/Tests/base/NSString/unichar_format.m
new file mode 100644
index 000000000..7e3f516c5
--- /dev/null
+++ b/Tests/base/NSString/unichar_format.m
@@ -0,0 +1,24 @@
+#import "ObjectTesting.h"
+#import <Foundation/NSAutoreleasePool.h>
+#import <Foundation/NSString.h>
+
+int main()
+{
+  NSAutoreleasePool	*arp = [NSAutoreleasePool new];
+  NSString              *narrow = @"aaaa"; // fits in a single byte
+  NSString              *wide = @"a\u20AC\u20ACa"; // Euro signs, requires UTF-16 storage
+  NSString		*narrowNarrowFormat = [NSString stringWithFormat: @"a%@a", narrow];
+  NSString		*narrowWideFormat = [NSString stringWithFormat: @"a%@a", wide];
+  NSString		*wideNarrowFormat = [NSString stringWithFormat: @"\u20AC%@\u20ac", narrow];
+  NSString		*wideWideFormat = [NSString stringWithFormat: @"\u20AC%@\u20ac", wide];
+  PASS_EQUAL(narrowNarrowFormat, @"aaaaaa",
+             "Formatting byte-width string into a byte-width string works.");
+  PASS_EQUAL(narrowWideFormat, @"aa\u20AC\u20ACaa",
+             "Formatting a 16 bit wide string into a byte-width string works.");
+  PASS_EQUAL(wideNarrowFormat, @"\u20ACaaaa\u20AC",
+             "Formatting a byte-width string into a 16 bit wide string works.");
+  PASS_EQUAL(wideWideFormat, @"\u20ACa\u20AC\u20ACa\u20AC",
+             "Formatting a 16 bit wide string into a 16 bit wide string works.");
+  [arp release]; arp = nil;
+  return 0;
+}