Acetylenic monomers containing indan and other groups (1a-m) were polymerized with TaCl(5)n-Bu4Sn catalyst to give high molecular weight polymers. Most polymers were soluble in common organic solvents including toluene and chloroform, and they afforded free-standing membranes by the solution casting method. The onset temperature of weight loss of the polymers were over 400 degrees C, indicating high thermal stability. Despite the absence of bulky spherical groups, polymethylated indan-containing polymer membranes showed extremely high gas permeability. In particular, the oxygen permeability coefficients of polymers having 1,1,3,3-tetramethylindan and either p-fluorophenyl or p,m-difluorophenyl groups (2b and 2e) reached 17 900 and 18 700 barrers, respectively, which are even larger than that of the most permeable polymer known, poly(1-trimethylsilyl-1-propyne).