The aggregation behavior of a charged amphiphilic graft copolymer, prepared from poly(styrene-co-methyl methacrylate-co-maleic anhydride) and poly(ethylene oxide) monomethyl ether, was studied using flow field-flow fractionation (flow FFF). This method determines the hydrodynamic size of aggregates. The polymer was shown to have a complex aggregation behavior in aqueous solutions containing salt. This was expressed in the formation of differently sized structures, from single polymer chains to large polymolecular aggregates. The polymer changed its aggregation ability in the presence of different salts and salt concentrations, as reflected by changes in hydrodynamic size. Three different salts were tested : potassium chloride, sodium sulfate, and sodium phosphate. Even at micromolar concentrations of sodium sulfate, a pronounced aggregation was observed, probably due to reduced charge repulsion. At higher concentrations of the "salting-out" salts, the hydrodynamic size increased rapidly, reflecting the reduced solubility of the hydrophilic parts of the polymer, i.e. the poly(ethylene oxide) grafts. Sodium sulfate, and sodium phosphate in particular, had a stronger influence on polymer aggregation than potassium chloride. This agrees with current knowledge of electrolytic effects on poly(ethylene oxide) solutions. Flow FFF was found to be a suitable characterization technique for these complex systems, rapidly and efficiently separating different polymer aggregate populations ranging from a few nanometers up to 0.1 mu m.