Poly[1-(trimethylsilyl)-1-propyne], PMSP, has the highest gas permeability of any synthetic polymer but exhibits a deterioration of these gas permeability properties with time. A technique of blending PMSP with liquid poly(dimethylsiloxanes), PDMS, of different molecular weights was used to alter the decay in gas permeability properties of PMSP. Sorption isotherms and permeability coefficient of CO2 were measured. The Langmuir capacity constant, C(H)’, decreased with increasing content of PDMS in the membrane. Both the sorption and the permeability behavior showed that modified membranes changed from a glassy state to a rubbery state where the PDMS content of the lower viscosity liquids was larger than 40 wt%. PDMS of higher viscosity was more effective for decreasing of C(H)’. These results suggest that the filling of larger PDMS molecules occurred selectively in the larger microvoids rather than the smaller ones and the filling of smaller PDMS molecules occurred in both the small and large microvoids in the PMSP membrane.