An investigation of the behavior of carbon-supported Pt and PtCo cathode electrocatalysts was carried out with the aim to evaluate their performance and resistance to degradation under high temperature (110−130 °C) operation in a polymer electrolyte membrane fuel cell (PEMFC) based on a new ionomer membrane (Aquivion). Nanosized Pt and PtCo catalysts with similar crystallite size (2.7−2.9 nm) were prepared by using a colloidal route. A suitable degree of alloying and a face-centered-cubic (fcc) structure were obtained for the PtCo catalysts by using a carbothermal reduction. The surface properties were investigated by X-ray photoelectron spectroscopy (XPS) and low-energy ion scattering spectroscopy (LE-ISS, 3He+ at 1 kV). The formation of a Pt skin layer on the surface of the alloy electrocatalyst was obtained by using a preleaching procedure. Furthermore, the amount of Pt oxides on outermost atomic layers was much smaller in the PtCo than in the Pt catalyst. These characteristics appeared to influence catalysts’ performance and degradation. Accelerated tests (electrochemical cycling) at 130 °C in a pressurized PEMFC showed a better stability for the PtCo alloy as compared to Pt. Furthermore, better performance was obtained at high temperatures for the preleached PtCo/C as compared to the Pt/C cathode catalyst. At a moderate pressure of 1.5 bar (abs), maximum power densities of 800 and 700 mW cm−2 at 110 °C (H2−O2) were achieved for the PtCo cathode with 50% and 25% relative humidity (RH), respectively, by using 0.3 mg Pt cm−2 loading. At high pressure of 3 bar (abs), a maximum power density exceeding 1000 mW cm−2 was obtained at 130 °C and 100% RH.