Novel one-step technique has been proposed for octyl-substituted polymethylsiloxane (POMS) synthesis and vulcanization. The technique makes it possible to prepare D(3)/D(1) selective gas separation membranes. The fact of POMS formation and vulcanization by hydrosilylation reaction involving 1-octene and polymethylhydrosiloxane has been confirmed by IR spectroscopy data. On the basis of the results obtained by measuring permeability of POMS membranes to permanent gases and propane, a preferred modifier to crosslinker ratio at which polymer films with the best gas transport properties can be obtained has been estimated at 95/5. Experimentally determined permeability coefficients for methane and propane (270 and 1560 barrer, respectively) and ideal propane/methane selectivity of 5.8 allow for the conclusion that POMS obtained in the study has transport properties comparable to those of similar membrane materials reported in the literature as obtained via multistep synthesis.