The permeation behavior of pure gases, binary and ternary gas mixtures of O-2, N-2 and CO2 through poly(dimethylsiloxane) (PDMS) membrane was investigated with different pressures and compositions of feed gas. An analytical model expressed in terms of pressure and feed composition was derived from permeation behavior of pure gases and binary gas mixtures to predict quantitatively the flux and composition of permeated streams. Furthermore, the model was extended to the application of the ternary case and the results showed a good agreement with the experimental data. It was indicated that the model could be used to evaluate the separation properties and to choose the optimal feed conditions for the membrane separation system of O-2, N-2 and CO2. (c) 2005 Elsevier B.V. All rights reserved.