Constitutive equations are derived for enthalpy recovery in a glassy polymer after quench from above the glass transition temperature T-g to a temperature Tin the sub-T-g region. The model is based on the trapping concept, which treats a disordered medium as an ensemble of cooperatively rearranging regions (CRR) hopping in potential wells as they are thermally activated. Rearrangement occurs when a CRR reaches some liquid-like energy level in a hop. The rate of hops is described by the theory of thermally activated processes, whereas the probability to change trap in a hop is determined by the difference between the current and equilibrium concentrations of cages. A nonlinear parabolic equation is developed for the distribution of traps. This equation is used to describe entropy recovery in amorphous and semicrystalline polymers. Fair agreement is demonstrated between experimental data for poly(ether imide), poly(ethylene terephthalate) and polystyrene and results of numerical simulation. Some phenomenological relations are suggested to predict the effect of temperature, molar mass and degree of crystallinity on material parameters.