The pyrolysis temperature is a critical factor in the synthesis of the silicon-carbon membranes which are derived from poly(dimethylsilane). These membranes show respectable molecular sieving capability when the pyrolysis temperature is 873 K or lower. Permeation of He, H-2, N-2, CH4, and i-C4H10 through these membranes is dominated by activated diffusion. Nonetheless, a pyrolysis temperature higher than 873 K sinters the micropores and reduces the integrity of the membrane. Consequently, both permeation and permselectivity decline. The 873 K pyrolyzed membrane exhibits H-2 permeance of 2.7 x 10(-9) mol Pa-1 s(-1) m(-2), H-2/N-2 selectivity of around 20, and H-2/i-C4H10 selectivity of around 80, at permeation temperature 473 K.