The literature presents different methods for quantifying and characterizing partially hydrolyzed polyacrylamide (HPAM) in solution. However, only a few of these are suitable for determining the critical overlap concentration (C*). The evaluation of C* is important for describing the transition from the dilute to the semidilute regime, that is, when the solution depicts a characteristic viscosity at concentrations above C*. This article describes the determination of C* for HPAM in solution by potentiometry. The molecular weight, degree of hydrolysis, and polymerization degree are determined by mathematical manipulation of the constant of formation of aggregates, which is based on the law of mass action. The potentiometric curves were initially linear; asymptotic behavior followed. The inflection point was determined by the intersection, and the resulting equation of mathematical development statistically satisfied the experimental data and described the number of moles of monomers, the equilibrium constant for the formation of the aggregate, C*, and the acidity constant of the polymer. The results for C*, the degree of copolymerization, and the molecular weight proved that this method is a good alternative for the characterization of polymers with ionizable monomers and that are soluble in water. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013