Running into this problem with my object tree, which I cannot modify (testing already written code).

The serialization goes up the tree until it encounters a GTKLookAndFeel object. This class extends SynthLookAndFeel which throws a NotSerializableException as part of its writeObject method. The SynthLookAndFeel class comments state:

 * This class implements {@link Serializable} as a side effect of it
 * extending {@link BasicLookAndFeel}. It is not intended to be serialized.
 * An attempt to serialize it will
 * result in {@link NotSerializableException}.

While this might not be very clean implementation for SynthLookAndFeel, it shouldn't prevent XStream to serialize the tree using a different converter, instead of the current behavior of throwing a ConversionException.