Abstract-Random acrylic acid/hydroxyethyl methacrylate (AA/HEMA) copolymers were synthesized to study the influence of non-ionic (HEMA) structure defects introduced in poly(acrylic acid) on the complex formation with PEO (fiW 23200) at pH 3-3.5 in aqueous solution. Complex formation as studied with potentiometric titration (concentration 0.01-0.03 M monomer residues) was increasingly impeded when the molar ratio of HEMA to AA units in the polymers increased. Copolymers of AA and HEMA with molar ratios 30: 1, 10 : 1 and 3 : 1 were used in this study. It was shown that in the copolymers the number of carboxylic acid groups taking part in complexation decreased with increasing amounts of HEMA in the copolymer. However, with increasing amounts of structure defects the amount of PEO per AA residue (at equilibrium complexation) increased, suggesting less efficient packing of PEO on the acrylic copolymer chain. Copyright 0 1996