The complexation of poly(acrylic acid) (PAA) with Cu2+ ions in a dilute aqueous solution has been investigated as a function of the mixing molar ratio of the two species and the neutralization degree (i) of PAA by means of turbidimetry, viscometry, potentiometry, and ultraviolet-visible (UV-vis) spectrophotometry. Turbidimetry reveals that, for i > 0.1, phase separation takes place when the mixing ratio approaches the critical value of two carboxylate ions per Cu2+ ion, which is indicative of the formation of a 2 : 1 polymer/metal complex. This complex is very compact, as evidenced by the very low reduced viscosity values obtained just before phase separation. The variation of the fraction of complexed carboxylate anions and complexed Cu2+ ions can be followed as a function of the mixing ratio and i from the analysis of the potentiometric results for i < 0.5. Finally, the combination of the potentiometric and UV-vis spectrophotometric results supports the idea that both mononuclear and binuclear PAA/Cu2+ complexes are formed in an aqueous solution, depending on the mixing ratio and i. (c) 2008-Wiley Periodicals, Inc.