Differential size-exclusion chromatography (SEC) is used to characterize complexes formed between gelatin and two synthetic polyelectrolytes, sodium poly(styrenesulfonate) and sodium poly(2-acrylamido-2-methylpropanesulfonate). The analysis is performed under aqueous, low-salt conditions where maximum complexation between gelatin and the polyelectrolytes occurs. The adsorption effects that are commonly encountered in conventional SEC for gelatin and other charged polymers chromatographed under these solution conditions are minimized, because the columns are constantly equilibrated with the analytes in the mobile phase. Analyte solutions of identical composition, but of higher or lower concentration than that contained in the mobile phase, are injected, resulting in positive or negative detector responses, respectively. This method can separate the complexes from individual components, and can be used to determine relative sizes and stoichiometries of the complexes as a function of both the input ratio of gelatin to polyelectrolyte and the molecular weight of the polyelectrolyte.