The stability of Pt-Co/C and Pt-Pd/C electrocatalysts relative to that of a commercial Pt/C catalyst was measured in terms of the loss of the electrochemical surface area (ESA). The electrocatalytic activity was investigated in an acidic solution (0.3 M H2SO4) and in a single PEM fuel cell under H-2/O-2 conditions. In the acidic solution, the ESA of the catalyst decreased as the number of repeated potential cycles increased, which is likely to be due to dissolution of the different metals contained within the catalyst structure. In the fuel cell environment, the deterioration of the cell performance increased as the number of repeated potential cycles increased. Thus, the loss of cell performance may be related to the loss of the ESA. In addition, the loss of the catalyst's ESA affected the cell performance at low-, medium-, and high-current densities, indicating a loss of either the activation potential or an ohmic loss. Among the three electrocatalysts evaluated, the Pt-Co/C based one exhibited the highest electrocatalytic activity in both the acidic solution and in the fuel cell environment.