In this work, a new polydimethylsiloxane (PDMS) membrane was synthesized and its sorption, diffusion, and permeation properties were investigated using H-2, N-2, O-2, CH4, CO2, and C3H8 as a function of pressure at 35 degrees C. PDMS, as a rubbery membrane, was confirmed to be more permeable to more condensable gases such as C3H8. The synthesized PDMS membrane showed much better gas permeation performance than others reported in the literature. Based on the sorption data of this study and other researchers' works, some valuable parameters such as Flory-Huggins (FH) interaction parameters, chi, etc., were calculated and discussed. The concentration-averaged FH interaction parameters of H-2, N-2, O-2, CH4, CO2, and C3H8 in the synthesized PDMS membrane were estimated to be 2.196, 0.678, 0.165, 0.139, 0.418, and 0.247, respectively. Chemical similarity of O-2, CH4, and C3H8 with backbone structure of PDMS led to lower chi values or more favorable interactions with polymer matrix, particularly for CH4. Regular solution theory was applied to verify correctness of evaluated interaction parameters. Local effective diffusion coefficient of C3H8 and CO2 increased with increasing penetrant concentration, which indicated the plasticization effect of these gases over the range of penetrant concentration studied. According to high C3H8/gas ideal selectivity values, the synthesized PDMS membrane is recommended as an efficient membrane for the separation of organic vapors from noncondensable gases. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 33-48, 2010