A copolymer prepared by the copolymerization of ethyl vinyl ether and maleic anhydride underwent amidoacidation with N,N-dimethylaminopropylamine. The obtained ethyl vinyl ether/N,N'-dimethylaminopropyl maleamidic acid copolymer was then reacted with methyl iodide to yield poly(methyl iodide quaternized ethyl vinyl ether/N,N'-dimethylaminopropyl maleamidic acid) (MIQEDMAPMA). The greatest difference from other polyelectrolytes was the carboxylic group on the polymer chain unit of MIQEDMAPMA. Its aqueous solution properties in various salts and at various pH values were studied by measurements of the reduced viscosity and intrinsic viscosity. The reduced viscosity and intrinsic viscosity of this cationic polyelectrolyte were related to the types and concentrations of the added salts. The tendency of the salt effect was similar to that of other polyelectrolytes; that is, soft salt anions were more easily bound to the quaternary ammonium (R4N+) of MIQEDMAPMA than hard salt anions. Some salt ions strongly attracted the quaternary ammonium of the cationic polymeric side chain for the agglomeration of the polymers. (C) 2003 Wiley Periodicals, Inc.