Abstract: The quantification of metal-chelating activity of caffeic and ferulic acids( 1 and 2, resp.) was successfully performed by using a potentiometric system with data-analysis computer programs. The method was applied to two phenolic models, which have been systematically reported as antioxidants. Although a chain-breaking mechanism was proposed, several studies pointed out the possibility of complexation of transition metals that can participate in single-electron reactions and mediate the formation of oxygen-derived free radicals. In this work, the complexation propertiestowardsCu II were investigated by potentiometry with a glass electrode. Acidity constants of the ligands (phenolic acids) and the formation constants of the ligandmetal complexes were evaluated by potentiometry. The modeling of the titration curvesand the data treatment were performed with the computer programsSuperquad and Bes t. A detailed quantitative examination of the complexation species formed in the Cu II /caffeic acid (1) and Cu II /ferulic acid (2) systems is presented together with the formation constants (log ). Results have shown that the complexation properties of the two phenolic acids towardsthe trans ition metal are quite different: the activity of caffeic acid ( 1) wasfound higher than that of ferulic acid (2). The data are important to get insight into the mechanism of action of antioxidants, and, in this case, could partially explain the efficacy of caffeic acid in the protection of LDL oxidative damage. In addition, the analytical method developed could be applied to quantify the chelating activity of important biological compounds, such as allopurinol, uric acid, cinnamic acids, flavonoids, and anthocianins, and, in that way, could be a valuable tool to understand the mechanisms underlying their protective effects.