The diffusivity, permeability, and solubility coefficients of O-2, N-2, CO2, and CH4 have been measured in PVC and in four samples of PVC functionalized with mereapto pyridine groups up to 6.8, 11, 21, and 46 mol %. It has been found that the permeability of all four gases progressively increases by a factor of more than 4 because of a similar increase in the diffusion coefficients. In addition, permeability selectivity augments significantly for the gas pair O-2/N-2 and only slightly for the pair CO2/CH4. The structural changes in the modified samples have been monitored by means of density and T-g determinations and FFV calculations, which are a measure of interchain spacing and chain stiffness. Am interpretation of the changes seen in the gas transport properties of modified PVC samples has been proposed in terms of the structural changes introduced by the progressive functionalization of the PVC chains.