I have got an object that is persisted by Hibernate and holds a Serializable property within (a complex structure of fine-grained objects which I don't want to store in separate database tables, anyway it's an 'composition' in UML terms).

The problem is that Hibernate does see changes in properties exposed as persistent, but the Serializable thing seems to be non-transparent. In other words, any setXXX() methods called on Serializable itself or on the objects it consists of are not visible to Hibernate and, as a consequence, changes are not persisted.

Is there a recommended way (smarter than what I do now, just explicitly calling update() methods on Hibernate's Session) to track changes within Serializable?

You are mistaken. Check the implementation of SerializableType. It is all handled transparently.

Yes indeed. You are right, Gavin.
Awfully sorry for bothering you.