AbstractThe statistical thermodynamics of the melt and the crystal are discussed by a unified approach which distinguishes between the two phases only in terms of a structure function minimizing the configurational free energy. The ensuing equations of state at atmospheric and elevated pressures are compared with typical experimental data for polymer and oligomer melts, and crystalline as well as semicrystalline systems, with satisfactory results. As to the glassy state, expansivity data clearly reflect nmot only the glass transition, but also sub‐glass relaxation processes. In the spirit of the preceding discussion of the equilibrium melt, quasi‐thermodynamic conditions are considered. Here the non‐equilibrium character is reflected in the dependence of the extensive thermodynamic functions on thermal history in addition to the ambient variables of state. This state is discussed in terms of modifications of the liquid state