The (COOH)(2) oxidation reaction was studied at potentials below which the oxygen evolution reaction (OER) takes place. Pt was found to catalyze the (COOH)(2) oxidation reaction more strongly than Au, while Ru did not display any activity toward the (COOH)(2) oxidation reaction. Furthermore, under rapid stirring conditions, the (COOH)(2) oxidation reaction using Pt electrodes was shown to be activation controlled. Therefore, the (COOH)(2) oxidation currents can be related to the electroactive Pt area, as shown for a range of polycrystalline, bulk metal Pt, and Pt powder electrodes. The Pt surface area for multicomponent catalyst systems can also be estimated by combining (COOH)(2) oxidation data with the charge needed to oxidize adsorbed CO to CO2 (COads charge), as shown for a range of Pt- and Ru-containing powder electrodes. In fact, the combination of the two methods [(COOH)(2) oxidation current and COads charge] can be used as an in situ probe to estimate the fraction of Ru in the metallic state in the potential region where CO is adsorbed provided the surface ratio of Pt vs. Ru is known. (C) 2003 The Electrochemical Society.