Experimental evidence is presented supporting the conclusion that the role of anions in the electrocatalyzed oxidation of formic acid at Pd is more than simply blocking reactive surface sites. The effects of anion adsorption were probed in current-control experiments by varying the composition of the supporting electrolyte and the ratio of the supporting electrolyte concentration to that of the solute. The results are fundamentally different from those obtained in potential-control experiments conducted on the oxidation of small, oxygenated organic molecules. In the latter experiments, changing the supporting electrolytes produced quantitative differences that can be interpreted solely in terms of the blockage of surface sites by the adsorbed anions. In the current-control experiments, switching electrolytes caused drastic changes in the behavior exhibited by the oxidation of formic acid. Several experiments established trends that are opposite to those that would be predicted if the only effect of anions was to block the surface sites.