Viscometric, pH, optical, and calorimetric titration studies of an alternating copolymer of maleic acid and indene in aqueous NaCl of 0.01 to 0.27Mat 15 to 35°C showed the conformational transition of a compact to an expanded coil upon ionization of the primary carboxyl groups. the thermodynamic parameters of the transition were calculated. the results indicate that the bulky and inflexible side chain in indene sterically interferes with the formation of the compact form of the copolymer, but stabilizes the low entropic hydrophobic region.