Permeability functions are developed for pure gases and mixtures of N-2, CH4, CO2, and O-2 as a function of driving pressure for polysulfone and silicone rubber membranes. The gas mixtures used in the measurements include N-2-O-2, CO2-N-2-O-2, and CH4-N-2-CO2. The functions are expressed in a Linear form in terms of species partial pressures. The functions are in good agreement with the measured data. Positive and large weights for the fastest permeating species, CO2, are obtained in the permeability functions for all species in polysulfone. The opposite is obtained for the slower permeating species in the polysulfone permeability functions. This result is consistent with the membrane behaviour, where permeability of faster species is reduced by presence of slower species. The species weights in the silicone rubber permeability functions are 1-5 orders of magnitudes smaller than the function constant. This behaviour is dictated by limited Variations in species permeability in silicone rubber.