Several sulfolanes such as 3-methylsulfolane, sulfolane, and 3-sulfolene were tested as modifiers in poly(trimethylsilylmethyl methacrylate) (PTMSMMA) and poly(trimethylsilyl propyne) (PMSP) to improve the selectivity of CO2. The gas permeabilities for the PTMSMMA-blend membranes containing high 3-methylsulfolane content were determined on a nonvacuum system in which the membranes started to be measured at their steady states at 30 degrees C; those for all the other membranes were determined in a vacuum system in which those membranes were measured after they reached their unsteady states at 30 degrees C. The PTMSMMA-blend membrane containing 40% 3-methylsulfolane was found to give the best separation of CO2 under the conditions in this study compared to all the PTMSMMA-blend membranes and the others prepared in our work; its ideal separation factors for CO2 over N-2 were above 40 and its permeability coefficients of CO2 increased to above 250 Barrer. The modifications of PMSP membranes by impregnating with sulfolane and blending with sulfolene were found to be effective in improving the selectivity for CO2 over N-2 for the PMSP membrane. The ideal separation factors for CO2 over N-2 for the modified PMSP membranes impregnated with 30% sulfolane and blended with 25% 3-sulfolene were improved to above 10 and 13, respectively.