A styrene-[NN-dimethyl (maleamidic acid) propyl ammonium propane sulfonate] (SDMMAAPS) copolymer was synthesized through an amidoacidation reaction of styrene-maleic anhydride (SMA) alternating copolymer with N,N-dimethylaminopropylamine (ring-opening reaction), which was then reacted with propane sultone. The effect of various salt solutions on the intrinsic viscosity of this ampholytic ADMMAAPS copolymer was investigated. The results showed that the effect of counter ions on the intrinsic viscosity of SDMMAAPS was not entirely similar to that of other zwitterionic poly(sulfobetaine)s. The greatest difference from other poly(sulfobetaine)s is the carboxylic group on the polymer chain unit of SDMMAAPS. The Huggins constants for SDMMAAPS in aqueous salt solutions, however, were also quite different from those of other sulfobetaine copolymers, such as styrene-N,N-dimethyl (maleimido propyl) ammonium propane sulfonate] (SDMMAPS) copolymer. (D 2003 Wiley Periodicals, Inc.