Index: src/OFMutableString.h
==================================================================
--- src/OFMutableString.h
+++ src/OFMutableString.h
@@ -103,10 +103,19 @@
 /**
  * Lower the OFString.
  */
 - (void)lower;
 
+/**
+ * Inserts a string at the specified index.
+ *
+ * \param str The string to insert
+ * \param idx The index
+ */
+- (void)insertString: (OFString*)str
+	     atIndex: (size_t)idx;
+
 /**
  * Removes the characters at the specified range.
  *
  * \param start The index where the deletion should be started
  * \param end The index until which the characters should be deleted.

Index: src/OFMutableString.m
==================================================================
--- src/OFMutableString.m
+++ src/OFMutableString.m
@@ -351,10 +351,32 @@
 - (void)lower
 {
 	[self _applyTable: of_unicode_lower_table
 		 withSize: OF_UNICODE_LOWER_TABLE_SIZE];
 }
+
+- (void)insertString: (OFString*)str
+	     atIndex: (size_t)idx
+{
+	size_t nlen;
+
+	if (isUTF8)
+		idx = of_string_index_to_position(string, idx, length);
+
+	if (idx > length)
+		@throw [OFOutOfRangeException newWithClass: isa];
+
+	nlen = length + [str cStringLength];
+	string = [self resizeMemory: string
+			     toSize: nlen + 1];
+
+	memmove(string + idx + [str cStringLength], string + idx, length - idx);
+	memcpy(string + idx, [str cString], [str cStringLength]);
+	string[nlen] = '\0';
+
+	length = nlen;
+}
 
 - (void)removeCharactersFromIndex: (size_t)start
 			  toIndex: (size_t)end
 {
 	if (isUTF8) {

Index: tests/OFStringTests.m
==================================================================
--- tests/OFStringTests.m
+++ tests/OFStringTests.m
@@ -273,10 +273,16 @@
 	    [[@"foo\"ba'_~$" stringByURLEncoding] isEqual: @"foo%22ba%27_~%24"])
 
 	TEST(@"-[stringByURLDecoding]",
 	    [[@"foo%20bar%22+%24" stringByURLDecoding] isEqual: @"foo bar\" $"])
 
+	TEST(@"-[insertString:atIndex:]",
+	    (s[0] = [OFMutableString stringWithString: @"𝄞öööbä€"]) &&
+	    R([s[0] insertString: @"äöü"
+			 atIndex: 3]) &&
+	    [s[0] isEqual: @"𝄞ööäöüöbä€"])
+
 	EXPECT_EXCEPTION(@"Detect invalid encoding in -[stringByURLDecoding] "
 	    @"#1", OFInvalidEncodingException, [@"foo%bar" stringByURLDecoding])
 	EXPECT_EXCEPTION(@"Detect invalid encoding in -[stringByURLDecoding] "
 	    @"#2", OFInvalidEncodingException,
 	    [@"foo%FFbar" stringByURLDecoding])