A reliable, predictive model for the solubility of gases and vapours in glassy polymers has been recently presented : it is based on the free energy expression for the polymer penetrant mixture as obtained from the lattice fluid theory and on the idea that the partial polymer density is an internal state variable for the system. When the polymer density for the sorption condition is known, the model can be used in a pure predictive way as the solubility is estimated on the basis of pure penetrant and pure polymer PVT properties. In this work the basic assumptions of the model and its development are discussed in detail. The pseudoequilibrium condition and the calculation of the penetrant solubility under pseudo-equilibrium conditions are clearly stated. In closure, several examples comparing model predictions and experimental results for the solubility in glassy polymers are presented. In all the cases considered, the comparison confirms the model capability to satisfactorily predict solubility data in nonequilibrium glassy polymers and supports the validity of the underlying physical model.