Index: src/OFMutableString_UTF8.m
==================================================================
--- src/OFMutableString_UTF8.m
+++ src/OFMutableString_UTF8.m
@@ -213,11 +213,11 @@
 
 	if (index > _s->cStringLength)
 		@throw [OFOutOfRangeException exception];
 
 	/* Shortcut if old and new character both are ASCII */
-	if (!(character & 0x80) && !(_s->cString[index] & 0x80)) {
+	if (character < 0x80 && !(_s->cString[index] & 0x80)) {
 		_s->hashed = false;
 		_s->cString[index] = character;
 		return;
 	}
 
@@ -244,11 +244,11 @@
 
 		_s->cStringLength -= lenOld;
 		_s->cStringLength += lenNew;
 		_s->cString[_s->cStringLength] = '\0';
 
-		if (character & 0x80)
+		if (character >= 0x80)
 			_s->isUTF8 = true;
 	} else if (lenNew < (size_t)lenOld) {
 		memmove(_s->cString + index + lenNew,
 		    _s->cString + index + lenOld,
 		    _s->cStringLength - index - lenOld);
@@ -255,10 +255,13 @@
 		memcpy(_s->cString + index, buffer, lenNew);
 
 		_s->cStringLength -= lenOld;
 		_s->cStringLength += lenNew;
 		_s->cString[_s->cStringLength] = '\0';
+
+		if (character >= 0x80)
+			_s->isUTF8 = true;
 
 		@try {
 			_s->cString = [self
 			    resizeMemory: _s->cString
 				    size: _s->cStringLength + 1];
@@ -344,14 +347,16 @@
 	if (encoding == OF_STRING_ENCODING_UTF_8)
 		[self appendUTF8String: cString
 				length: cStringLength];
 	else {
 		void *pool = objc_autoreleasePoolPush();
+
 		[self appendString:
 		    [OFString stringWithCString: cString
 				       encoding: encoding
 					 length: cStringLength]];
+
 		objc_autoreleasePoolPop(pool);
 	}
 }
 
 - (void)appendString: (OFString*)string
@@ -640,10 +645,17 @@
 		_s->cString = [self resizeMemory: _s->cString
 					    size: newCStringLength + 1];
 
 	_s->cStringLength = newCStringLength;
 	_s->length = newLength;
+
+	if ([replacement isKindOfClass: [OFString_UTF8 class]] ||
+	    [replacement isKindOfClass: [OFMutableString_UTF8 class]]) {
+		if (((OFString_UTF8*)replacement)->_s->isUTF8)
+			_s->isUTF8 = true;
+	} else
+		_s->isUTF8 = true;
 }
 
 - (void)replaceOccurrencesOfString: (OFString*)string
 			withString: (OFString*)replacement
 			   options: (int)options
@@ -720,10 +732,17 @@
 	[self freeMemory: _s->cString];
 	_s->hashed = false;
 	_s->cString = newCString;
 	_s->cStringLength = newCStringLength;
 	_s->length = newLength;
+
+	if ([replacement isKindOfClass: [OFString_UTF8 class]] ||
+	    [replacement isKindOfClass: [OFMutableString_UTF8 class]]) {
+		if (((OFString_UTF8*)replacement)->_s->isUTF8)
+			_s->isUTF8 = true;
+	} else
+		_s->isUTF8 = true;
 }
 
 - (void)deleteLeadingWhitespaces
 {
 	size_t i;