Flow field-flow fractionation (FlFFF) was used to characterize the aggregation of a charged, amphiphilic copolymer, MMA 550, prepared from poly(styrene-co-methyl methacrylate-co-maleic anhydride) and poly(ethylene oxide) monomethyl ether. MMA 550 has a charged hydrophobic backbone and hydrophilic grafts which give the polymer a complex behavior in aqueous solution. FlFFF turned out to be a powerful tool for separation and size determination of different polymer aggregate populations in various media. The hydrodynamic diameter of the polymer was obtained from FlFFF measurements in water and pH-adjusted solutions. In pure water, the majority of the polymer molecules had a hydrodynamic diameter of 3-4 nm, which was also found at higher pH values (pH 9), When the pH was lowered, the hydrodynamic diameter increased rapidly. This suggested the formation of large micelles, probably polymolecular, as an effect of the reduced charge repulsion.