Nonideal effects are analyzed for the separation of C02from tertiary gas mixtures by polymeric membranes. The analysis includes comparison between the behaviour of ideal and nonideal gas properties during the separation process. Separation characteristics are made as function of feed composition, pressure difference across the membrane, and membrane area. Gas flow in the feed and permeate compartments is modelled by both plug flow and complete mixing models. Results show that the nonideal behaviour of C02affects the separation characteristics and exhibits large deviations from the predictions of the ideal gas law. This behaviour is more pronounced at high feed pressures and large C02mole fraclions in the feed stream.