We have studied the effect of several experimental variables (CO admission time, bubbling rate of CO, and magnetic stirring of the electrolyte) on the electrooxidation at low potentials of dissolved CO on Pt. Although this electrooxidation was affected by changes in the said variables, in ail cases it was always accompanied by the presence in the same potential range of a hump of chemisorbed CO (here designated as CO(1)) in parallel CVs recorded in the absence of dissolved CO and whose charge was about 15% of a monolayer of chemisorbed CO. Since this hump of chemisorbed CO(1) preceded the main peak of electrooxidation of chemisorbed CO, it is clear that electrooxidation of the dissolved CO takes place only on this 15% of the Pt surface free from chemisorbed CO(1). The stationary currents, the peak currents, and the Cottrell plots of dissolved CO electrooxidation were in good agreement with those calculated for a wholly active surface. The only possible explanation of this behavior is that the CO(1)-free Pt sites act as an ensemble of microelectrodes, so that the overlap of their hemispherical diffusion layers yields the same current density as a conventional electrode. Actually, both plots of peak potential vs the logarithm of the sweep rate and of peak current density vs the square root of the sweep rate yielded the linear behavior expected of conventional electrodes, which shows that the time necessary for both electrooxidation of the chemisorbed CO(1) and for the said overlap was negligible for sweep rates up to 10 V s(-1). From this an upper value of about 3 mu m for the diameter of the CO-free patches was estimated.