In this study, composite PDMS/ (PEI membrane was synthesized by phase inversion method. The permeation and sorption of hydrogen and methane was measured at different conditions. The obtained results are presented as below: Critical concentration of PEI membrane was determined by viscometric method and it was 24%. Pressure tests indicated that by increasing the trans-membrane pressure from 1 to10 bars, the H2 permeance descended but CH4 permeance was enhanced. This led to selectivity reduction by increasing the pressure. Solubility of H2 declined and solubility of CH4 increased by...