The gas transport coefficients of polyvinyl chloride (PVC) and PVC modified with pyridine groups have been studied. It has been observed that there is a strong time dependence of the permeability and diffusivity of oxygen, nitrogen, carbon dioxide and methane in membranes prepared by solvent casting of PVC and pyridine modified PVC. For PVC there is a two-fold reduction of the diffusion coefficients during the first two days, and about one order of magnitude, a month after the membranes are prepared, and no stabilisation of the trend is seen after a month. Membranes prepared from modified PVC show a short-term diffusion rate reduction which is similar to that found in PVC, while at longer times the diffusion rate decrease levels off quickly, attaining constant values after about ten days. The time dependence of the transport coefficients is attributed to the samples' physical aging and an attempt is made to fit the experimental data by considering a stretched exponential time dependence of the volume contraction on aging. (C) 2001 EIsevier Science Ltd. All rights reserved.