Nuclear magnetic resonance (NMR) spectroscopy has been used in conjunction with voltammetry to investigate the mechanism for the electrocatalytic oxidation of CO on a carbon-supported PC fuel cell electrocatalyst in 2 M H2SO4 at ambient temperature. The states of adsorbed CO(COads), the conditions which give rise to them, and their oxidation pathways were investigated as a function of COads coverage and the working electrode potential of adsorption (E-ads). CO was adsorbed from the electrolyte at various fixed values of E-ads between 0 and 600 mV vs. RHE. Submonolayer coverages of COads were formed in two different ways, by adsorption for limited amounts of time and by partial oxidation of an initially saturated coverage. The COads coverage was then characterized via voltammetric oxidation and via C-13 NMR. Voltammetry data were analyzed qualitatively in terms of the number, shape, and position of COads oxidation peaks and quantitatively in terms of the total charge required to oxidize the COads coverage and the total number of catalyst sites occupied by the COads. NMR spectra were analyzed qualitatively in terms of their lineshape and position and quantitatively in terms of their integrated intensity. These results have been utilized in the formulation of an overall model for the adsorption and oxidation of CO on Pt in an acid electrolyte at ambient temperature. Three distinct forms of COads are identified on the Pt electrocatalyst, a linear carbonyl, a bridged carbonyl, and a reduced carbonyl (a hydrogenated carbonyl requiring a three-electron transfer for complete oxidation to CO2). The relative populations of these three species are found to depend strongly on E-ads and to be substantially independent of coverage. Different oxidation behaviors are observed for different populations of the three COads species, suggesting that different oxidation reaction pathways are associated with the three COads states. (C) 2001 The Electrochemical Society. All rights reserved.