Poly(ethylene glycol) monomethyl ethers (MPEG) were grafted on copolymers of maleic anhydride and styrene, methyl methacrylate, and ethylhexyl methacrylate, respectively. Hydrolysis of the remaining anhydride residues gave graft copolymers carrying a large number of carboxylic acid groups along the main chains. The properties in aqueous solutions of these graft copolymers were studied with respect to aggregation behavior and viscometric properties, Aggregation of the polymers was examined by quasi-elastic light scattering and now field-flow fractionation in water and KCl solution. Both methods showed that the anionic graft copolymers mainly were present as single molecules in pure water, with a minor fraction of aggregates. In KCI solution, aggregates with average sizes of approximately 30 nm were the dominant species. In aqueous solution, the polymers exhibited polyelectrolyte behavior, i.e., a dramatic increase of the viscosity upon neutralization. Graft copolymers with hydrophobic groups in the backbone had lower viscosities.