Abstract
The effects of the internal energy at the glass transition are studied by means of the Prigogin-Defay ratio. It is first shown that the jumps of the heat capacities at constant pressure and at constant volume must be comparable in magnitude. In addition, special systems are studied, where the Prigogine-Defay ratio turns out to depend only on the heat-capacity jumps. Finally, it is shown that the kinetic nature of the glass transition is necessarily related to an excess of internal energy in the melt.