Structural relaxation for simple and more complex thermal histories is described by a phenomenological model based on a non-exponential
relaxation function, the reduced-time concept and the nonlinear structural contribution to the relaxation time. The history,
development of experimental techniques and data analysis is described. It is shown that the volume and enthalpy relaxation
response can conveniently be compared on the basis of a fictive relaxation rate, Rf. A simple equation relating Rf and the parameters of the phenomenological model is given. The calculated data for moderate departures from equilibrium are
in good agreement with our experiments and data previously reported in the literature.