Substitution of thiourea to poly(diphenylacetylene) having 2-bromoethoxy groups (2a) gave an sulfonium-containing poly(diphenylacetylene) [3a (Br (-) )]. The counteranions of 3a (Br (-) ) could be exchanged using CF3COOK and (CF3SO2)(2)NLi, and the polymers 3a (TFAc (-) ) and 3a (Tf (2) N (-) ) were obtained. Polymer 3a (Br (-) ) was hydrolyzed with NaOHaq to afford the polymer 4a possessing mercapto groups. The CO2 permeability coefficients () and the separation factor () of 3a (Br (-) ) were 15 barrers and 16, respectively. The selectivity was low compared to the analogous polymer, poly(diphenylacetylene) having imidazolium salts (), because 3a (Br (-) ) interacted strongly with CO2, and the diffusion of CO2 was suppressed. The and of 3a (TFAc (-) ) were 43 barrers and 24, respectively, and those of 3a (Tf (2) N (-) ) were 52 barrers and 26, respectively. Both permeability and selectivity increased as the counteranion became bulkier. The mercapto group containing poly(diphenylacetylene) (4a) showed a relatively high CO2 permeability ( barrers).