If you have a class Decorated with DataContractAttribute, with a property of type ObjectId decorated with a DataMemberAttribute, and you return it as a response from a call through webHttpBinding (JSON), the property gets serialized as such:

"Id":

{"__type":"ObjectId:#MongoDB.Bson","machinePidIncrement":-5429670957678174237,"timestamp":1289430110}

I think it would make more sense if ObjectId (and BsonObjectId) would be custom serialized to just 4cdb247be076000000001f86 or maybe ObjectId('4cdb247be076000000001f86') for interoptability purposes.