CO electrooxidation on Au(111), Au(100) and Au(110) electrodes in 0.1 M HClO(4) and 0.1 M NaOH solutions has been studied by means of voltammetric measurements with hanging meniscus rotating bead-type electrodes. It is found that the reaction order in CO concentration is close to unity at potentials close to the onset of CO oxidation, and then it becomes higher than unity at higher potentials for all studied surfaces except for Au(111) in 0.1 M HClO(4). This behavior indicates that CO oxidation on gold takes place through a self-promotion mechanism, in which the presence of CO on the gold surface enhances the adsorption of its own oxidant. Specifically, this mechanism offers a plausible explanation for the higher catalytic activity in alkaline solutions, since CO adsorption is stronger under these conditions, and thus it can lead to a higher enhancement of the adsorption of the oxidant species.