Efforts to relate thermodynamic equilibrium and nonequilibrium phenomena have a long‐standing history. Examples of the latter include transport processes and relaxations to a new equilibrium state in response to an imposed perturbation. From the point of view of theory, the ultimate goal is a nonequilibrium statistical mechanics with a proper equilibrium limit. Attempts in this direction date back to van der Waals' times. For low molecular weight fluids, some success has been achieved by correlating semiempirically equation‐of‐state properties and viscosity. The starting point for our considerations is an equilibrium theory. Its central feature is an excess free volume function with known temperature and pressure dependence, which serves as a measure of disorder in a lattice model. This function moreover serves as a connecting link to the glass, generated from the melt by a specified formation history. Two aspects are under consideration. One is the initial response of the system to an imposed perturbation before time phenomena become observable. The second is the relaxation of thermodynamic and other properties. This may be viewed from a correlative point of view. That is, the measurement of one set allows the prediction of the other, time‐dependent set. Here the free volume function can be shown to provide the basis for the correlation. The other point of view involves kinetic theory, which treats volume relaxation as a gradual collapse of free volume. The results obtained in this context, and those following from probe studies in aging glasses by electron spin resonance, indicate the significance of our free volume function as a measure of molecular mobility in condensed systems.