We investigated the catalytic activity of gold particles toward CO electrooxidation by cyclic voltammetry and found that the formation of oxidic gold species on the surface of gold nanoparticles (AuNPs) is a necessary condition for electrooxidation of CO. The maximum potential in the positive-going sweep has a great effect on the formation of gold oxides as well as on the activity of AuNPs. We report the support-dependent electrooxidation of CO in aqueous solution as the first example of the contribution of the support to the high catalytic activity of AuNPs in electrochemical systems. We also show that particle size, reported to be a dominant factor in the catalytic oxidation of CO on AuNPs in solid-gas reaction systems, is no longer a critical factor controlling the activity of AuNPs in electrochemical systems. We demonstrate that indium tin oxide-supported gold submicroparticles, which are believed to be catalytically inactive due to their "large" size, exhibit surprisingly high activity toward electrooxidation of CO. (c) 2007 Elsevier Inc. All rights reserved.