Kinetic curves for sorption/desorption cycles have been generated for tetrafluoroethylene/propylene copolymer membranes at 25, 40, 55 and 70 degrees C by a gravimetric method. Diffusion coefficients have been obtained from the mathematical relations derived from Fick＇s relation. Numerical scheme based on the finite difference method has been used to describe the sorption/desorption processes. Theoretical kinetic curves have been compared well with the experimental values before 55% equilibrium saturation.