Abstract The NELF (non-equilibrium lattice fluids) model for the predictive calculations of the solubility isotherms of gases and vapors in glassy polymers is considered here with particular attention to its simplified formulation valid for the low pressure range. Its results are compared with the experimental data for several polymer penetrant couples. As input data, the simplified NELF model requires the knowledge of the P–V–T characteristic properties of the pure polymers and penetrants and, in addition, the density of the pure unpenetrated polymeric glass. In the low pressure range, the solubility calculations can thus be performed based on information typically available for a broad variety of systems, and without the need of any specific knowledge on the matrix dilation during sorption or desorption; the latter is on the contrary required in general by the complete NELF model, and is needed at higher pressures, when the swelling effects become significant. For the systems inspected, the low pressure approximation offers a satisfactory estimate of the solubility isotherms up to at least 0.3–0.5 MPa also for swelling penetrants like CO 2, while for much higher pressures the dilation effects need to be taken into account. For the case of negligible swelling effects (e.g. N 2 sorption) the approximation considered holds true in the entire pressure range inspected.