The separation characteristics of silicone rubber membranes are determined for CO2-N2 gas mixtures. The analysis is performed as a function of composition, flow rate and pressure of the feed gas. Results are presented in terms of the variation in component permeability and separation factor as a function of the above parameters. Component permeabilities are calculated using the complete mixing model. Data analysis over the studied pressure range shows that the permeability coefficient of pure CO2 gas in silicone rubber is 15 times higher than that of pure N2 gas. This behaviour is completely altered for a mixture of the gases, where the calculated separation factors at low feed pressures and low CO2 mole fractions in the feed stream are two- to three-fold lower than the separation factors for the pure gases. At higher feed pressures and high CO2 mole fractions in the feed stream, the above behaviour is reversed; the separation factors for the gas mixture are now higher than those for the pure gases. Comparison of the permeation characteristics of silicone rubber and cellulose acetate membranes for CO2-N2 gas mixtures shows similar ranges and values for the gas permeabilities and separation factors. However, much higher separation factors are obtained for the cellulose acetate membrane in the case of pure gas permeation.