The gas and vapor transport into films of Hyflon (R) Ion H, a short-side-chain perfluorosulfonic acid ionomeric (PFSI) membrane, suitable for use in proton exchange membrane fuel cells (PEMFC), has been studied at various temperatures (35 degrees C, 50 degrees C, and 65 degrees C). The permeability and diffusivity values of He, N-2, and O-2 show an Arrhenius type dependence on temperature in the range inspected. Pure water vapor permeation was studied at 65 degrees C, at low/medium activity values. The determination of water transport parameters has been performed by solving numerically the water mass balance with a variable diffusion coefficient and accounting for a water immobilization reaction onto the hydrophilic sites of the matrix. The boundary conditions vary in time according to the mass balance on the penetrant volume. The solution allows to represent closely the experimental permeation behavior in all its stages.