Index: src/OFUUID.m
==================================================================
--- src/OFUUID.m
+++ src/OFUUID.m
@@ -221,12 +221,12 @@
 }
 
 - (OFString *)UUIDString
 {
 	return [OFString stringWithFormat:
-	    @"%02X%02X%02X%02X-%02X%02X-%02X%02X-%02X%02X-"
-	    @"%02X%02X%02X%02X%02X%02X",
+	    @"%02x%02x%02x%02x-%02x%02x-%02x%02x-%02x%02x-"
+	    @"%02x%02x%02x%02x%02x%02x",
 	    _bytes[0], _bytes[1], _bytes[2], _bytes[3],
 	    _bytes[4], _bytes[5], _bytes[6], _bytes[7],
 	    _bytes[8], _bytes[9], _bytes[10], _bytes[11],
 	    _bytes[12], _bytes[13], _bytes[14], _bytes[15]];
 }

Index: tests/serialization.xml
==================================================================
--- tests/serialization.xml
+++ tests/serialization.xml
@@ -1,10 +1,10 @@
 <?xml version='1.0' encoding='UTF-8'?>
 <serialization xmlns='https://objfw.nil.im/serialization' version='1'>
   <OFMutableDictionary>
     <key>
-      <OFUUID>01234567-89AB-CDEF-FEDC-BA9876543210</OFUUID>
+      <OFUUID>01234567-89ab-cdef-fedc-ba9876543210</OFUUID>
     </key>
     <object>
       <OFString>uuid</OFString>
     </object>
     <key>