An algebraic semantics of object-oriented data models is given in the paper. Particular attention is paid to the following features typical of the object-oriented programming paradigm: complex muta e and immutable objects, object identity, object sharing, object types and type categories, inheritance, overloading, overriding, and late binding. The tools proposed in the paper can serve as a me anism for the formal definitions of different data models and as a basis for the development of future data models.