Java classes may define a specialized serialization (see JavaDoc for Serializable) by defining two methods: private void writeObject(ObjectOutputStream out) throws IOException private void readObject(ObjectInputStream in) throws IOException XStream is not capable of handling these cases. A simple usecase: public class SpecialClass { protected transient String[] array = ... private void writeObject( ObjectOutputStream out ) { // do something special with "array" here (i.e. encrpyt it) // and write to "out" } } XStream does not look at "array" because it's transient, but does also not treat writeObject(), so a serialized object of SpecialClass will be empty.