This study reports selectivities and permeances of pure nitrogen, oxygen, ethylene, ethane, propylene and propane and their mixtures through composite poly(dimethylsiloxane) (PDMS)-polysulfone membrane at ambient temperature. It was observed that both propylene and/or propane significantly plasticized PDNIS coating in pure as well as mixed gas permeation experiments. Above the plasticization pressure, the permeance order was P-C3 > P-C3 > P-C2 > P-C2 > P-O2 > P-N2, which matched the solubility order. However, permeance order was changed to P-C3 > P-C3 > P-C2 > P-C2 > P-O2 > P-N2 below the plasticization pressure, showing that propylene was more permeable than propane. Furthermore, plasticization caused coupling effects for ethylene, ethane and nitrogen in the presence of propylene and propane. Crown Copyright © 2005 Published by Elsevier B.V. All rights reserved.