Knowledge of Gibbs free energies of dissolution is needed for the borosilicate glasses used to store nuclear waste in order to model their alteration by aqueous solutions. In this paper, we first recall that, despite the fact that glasses are nonequilibrium substances, these data can be determined experimentally from a comprehensive set of measurements. Special attention is paid to configurational entropy, which cannot be determined by calorimetric methods, but has an important contribution to Gibbs free energies of formation. The results obtained for a series of simple and complex glasses are then presented. We finally discuss the composition dependence of the thermophysical properties of sodium borosilicates, whose practical importance is not restricted to the issue of nuclear waste storage.