Diphenylacetylenes having a diisopropylphenoxysilyl group [PhOSi(i-Pr)(2)C6H4C equivalent to C6H4R; R = H (1a), SiMe3 (1b)] were synthesized and then polymerized with TaCl5/n-Bu4Sn to provide the corresponding poly(diphenylacetylene)s (2a and 2b). These siloxy group-containing polymers afforded tough free-standing membranes by casting from toluene solutions. Treatment of the membranes (2a and 2b) with n-Bu4N+F- gave the silanol-functionalized poly(diphenylacetylene)s (3a and 3b). FT-IR spectra of the silanol-functionafized polymers revealed that some their silanol groups were consumed and the polymers were cross-linked via siloxane bond. The oxygen in DMF lowered membrane densities, and consequently their gas permeability significantly increased. The oxygen permeability coefficients of silanol group-containing membranes (3a and 3b) were 210 and 1300 barrers, respectively. These silanol-functionalized poly(diphenylacetylene) membranes were found to exhibit very high gas permeability irrespective of the presence of the polar silanol groups in the polymers. Membrane of 3a showed relatively high CO2 permselectivity as well as high CO2 permeability.