Constitutive equations are derived for enthalpy recovery in amorphous glassy polymers. The model is based on the concept of cooperative relaxation which treats a polymer as an ensemble of flow units rearranging at random times. The rate of rearrangement is determined by the Eyring theory of thermally activated processes. Fair ageement is demonstrated between results of numerical simulation and experimental data for polycarbonate, poly(ether imide), poly(methyl methacrylate), polystyrene and a glycerol glass.