Membranes based on dimethyl and methyl-phenylsiloxane were synthesized and then fully characterized. Transport properties and mechanical behavior were determined as a function of different methyl/phenyl group ratios and the effects of these groups on results were established. Membrane transport properties were evaluated from n-hexane absorption studies. Static tensile stress-strain deformation measurements, stress-hysteresis determination, and stress-relaxation studies were performed to assess the mechanical behavior of these membranes. The results indicate that both physical-mechanical and transport properties are strongly affected. by the methyl group content. The increase of the phenyl groups in the polymer chain impairs mechanical properties as a result of which decrease in crosslinking degree. and gas permeability due to the diminution in free volume and penetrant solubility.