Kinetic data are presented for the electrooxidation of aqueous solution carbon monoxide to carbon dioxide on two monocrystalline gold surfaces, Au(210) and (110), with the objective of elucidating the reaction mechanism, especially regarding the nature of adsorbed intermediate(s). Tafel plots (i.e., log rate versus electrode potential) were obtained by means of linear sweep voltammetry, particularly as a function of the solution reactant concentration and over a wide range (0-13.5) of the electrolyte pH. Under most conditions, the reaction order in CO was found to be near unity, as anticipated from the low coverages of adsorbed CO ascertained from infrared spectroscopy. Interestingly, the log rate-pH dependence observed on both surfaces display three distinct regions. At low (less than or equal to 2) and higher (greater than or equal to 4) pH values, essentially unit slopes were obtained (i.e., a unity reaction order in [OH-]), these regions being separated by one displaying apparently pH-independent kinetics. The potential region over which conveniently measurable electrooxidation kinetics occur lies substantially (ca. 0.8 V) below the onset of gold surface oxidation throughout the entire pH range. The pH-dependent kinetic behavior is consistent with a reaction pathway featuring the involvement of an adsorbed hydroxycarbonyl intermediate. While such intermediates have been identified in a number of metal complex-catalyzed CO oxidations in homogeneous solution, they apparently have not been considered previously for such electrocatalytic processes. The observed unity hydroxide reaction order at higher pH values is indicative of a rate-determining step (rds) involving OH- discharge onto adsorbed CO sites to form the hydroxycarbonyl species, while the apparent transition to zero-order kinetics at lower pH is consistent with water rather than OH- becoming the preferred reactant. This picture is supported by solvent isotope measurements which display the onset of a substantial H/D isotope effect below pH 4, signaling the occurrence of proton transfer within the rds. The emergence of another pH-dependent reaction pathway at the lowest pH values is attributed to a rds involving hydroxycarbonyl decomposition to form CO2. The mechanistic opportunities provided by the analysis of electrocatalytic rate-potential data over wide pH ranges are pointed out, along with the possibility that the proposed hydroxycarbonyl pathway occurs for a wide range of related processes on transition-metal surfaces.