We have prepared three polydirnethylsiloxane (PDMS) membranes by adjusting the mixing ratio of PDMS precursors-cross-linkable siloxane oligomer with vinyl groups and siloxane oligomer with silane groups-in the presence of Pt catalyst, and examined the separation performance of the resulting membranes for C2H4 and N-2. We have also investigated the sorption and permeation behavior of a series of olefins (e.g., C2H4, C3H6, and C4H8) and nitrogen (N2) over a pressures range of 1-20 atm and a temperature range of -20 to 50 degrees C with the PDMS membranes prepared from the mixing ratio of 10: 1. It was found that the solubility of olefins and their solubility selectivity over nitrogen were influenced strongly by the critical temperatures of the permeating gases and were enhanced greatly by increasing pressure or decreasing temperature. The permeability of olefins and their perm-selectivity over nitrogen in PDMS membranes showed critical-temperature-dependent rather than size-dependent order, over the operational ranges of pressure and temperature. With increasing feed pressure and/or decreasing temperature, the permeability of more condensable olefin gases increased, but nitrogen permeability changed only slightly and thus, olefin/nitrogen selectivity increased greatly. (c) 2007 Elsevier B.V. All rights reserved.