Phenylacetylenes having one or two trimethylsilyl groups at their benzene ring were synthesized, and polymerized by [Rh (cyclooctadiene) (PPh3)2]PF6, [Rh(norbornadiene) Cl]2, or WCl6 to afford high molecular-weight polymers in high yields. These poly(phenylacetylene)s were soluble in many kinds of solvents and were fabricated to tough membranes by the solvent casting method. The oxygen permselectivities of these membranes were very good. The oxygen permeability coefficients (P(O2)) and oxygen separation factors (alpha = P(O2)/P(Na)) of poly[2,4(op)-bis(trimethylsilyl)phenylacetylene] [poly(o-1-p-1)] and poly[(4(p)-trimethylsilyl)phenylacetylene][poly(p-1)] membranes were 4.73 X 10(-8) cc (STP) cm/cm2 s cmHg and 2.65, and 1.52 X 10(-8) cc (STP) cm/cm2 s cmHg and 3.39, respectively. In the case of poly(o-1-p-1), P(O2) was comparable to that of polymethylsiloxane (PDMS) and alpha was higher than that of PDMS. However, the P(O2) value reduced to 48% of its initial value in about 1 year. In the case of poly (p-1), the P(O2) value did not change in about 1 year. Ethanol permeated preferentially through these membranes (alpha(EtOH) > 1) in pervaporation of aqueous ethanol solution, whereas poly(phenylacetylene) [poly(PhA)] showed water permselectivity (alpha(EtOH) < 1). These favorable effects of trimethylsilyl groups on the oxygen and ethanol permselectivities were discussed on the basis of comparison with those of poly (PhA), other poly (substituted phenyl-acetylene) s, and trimethylsilyl-group containing polystyrenes.