The effects of the concentration of H2SO4 ([H2SO4]), which is the major decomposition product of polymer electrolyte membranes during the operation of fuel cells, on the performance of stabilized Pt skin/PtCo alloy nanocatalysts supported on high-surface-area carbon (PtxAL-PtCo/C) were investigated. Kinetically controlled activities for the oxygen reduction reaction (ORR) and the H2O2 yields ( P(H2O2)) on the PtxAL-PtCo/C were examined based on hydrodynamic voltammograms in O2-saturated 0.1 M HClO4 + X M H2SO4 ( X = 0 to 5 × 10-2) by use of the channel flow double electrode method at temperatures between 30 and 80 °C. At X ≤ 10-6 (1 μM) and all temperatures examined, the apparent ORR rate constants kapp@0.85 V (per unit electrochemically active surface area) on PtxAL-PtCo/C at 0.85 V vs the reversible hydrogen electrode (RHE) were nearly identical with those in sulfate-free 0.1 M HClO4 and were at least twice as high as those on a commercial Pt/C catalyst (c-Pt/C). The values of kapp@0.85 V on both PtxAL-PtCo/C and c-Pt/C decreased linearly with log[H2SO4] in the concentration range 10-6 < X ≤ 5 × 10-2. The detrimental effect by H2SO4 was less pronounced on PtxAL-PtCo/C than on c-Pt/C at high temperatures; the kapp@0.85 V value at X = 5 × 10-2 on the former at 80 °C was maintained as high as 87%, whereas that of the latter was 66% (34% loss). The values of peroxide production percentage P(H2O2) on PtxAL-PtCo/C at 80 °C were nearly constant (ca. 0.22% at 0.76 V vs RHE) up to X = 5 × 10-2. These superior characteristics are ascribed to weakened adsorption of sulfate on the Pt skin surface, supported by DFT calculations, which provides the great advantage of robustness in the presence of impurities, maintaining active sites for the ORR during the PEFC operation.