An electrochemical quartz crystal microbalance (EQCM) has been applied to investigate a CO-tolerance at Pt-Fe alloy electrodes, which exhibit excellent H-2 oxidation in the presence of 100 ppm CO. It was found that the Pt-Fe electrodes were electrochemically stabilized by a formation of a few monolayers of Pt-skin layer during several repetitive potential sweeps in 0.1 M HClO4 solution, i.e. a dissolution of alloy components followed by a rearrangement of thin Pt layer. In a positive-going potential scan at CO-adsorbed (theta(CO) = 1) electrodes of pure Pt and Pt-Fe alloy, a pre-oxidation peak was observed at 0.3-0.5 V versus RHE before a major CO oxidation peak at 0.7 V. Based on EQCM results, each pre-peak was assigned to an oxidation of weakly adsorbed CO at Pt-Fe alloy and adsorbed carboxylate at pure Pt, respectively. This is consistent with our previous results by in situ FTIR, and strongly supports a modified electronic structure of the Pt-skin from that of pure Pt, which has been proposed by us.