DetachedCriteria how it implemented now could not be used in application that uses wc for communication. The reason is that it uses CriteriaImpl class which has one of it field . Default wcf serializer DataContractSerializer does not support serializing System.Type. System.Type does not have any meaning outside of .net environment. Possible solution could be to change System.Type before serializing (OnSerializing event - it works for this serializer) for some other object and after deserializing (OnDeserialized event) transform this object back to System.Type. I think it is quite important now for NH to work with wcf because MS is deprecating remoting in favor of wcf.