The formation of polyelectrolyte complexes of linear copolymers and hydrogels based on copolymers of 2-[(methacryloyloxy)ethyl]trimethylammonium chloride with N-isopropylacrylamide (MADQUAT-NIPAAM) and poly(acrylic acid) (PAA) has been studied. The composition of the copolymer has been found to affect the composition of the polyelectrolyte complexes significantly, and the molecular weight of PAA influences their aggregation stability. Hydrogels of MADQUAT-NIPAAM immersed in solutions of PAA undergo contraction because of the formation of gel-polymer complexes. The rate of contraction and the final swelling degree of the gel-polymer complexes depend on the concentration of PAA in solution and its molecular weight. (C) 2004 Wiley Periodicals, Inc.