This paper aim to discuss a proper measure of the electrochemical active area of carbon supported Pt nanoparticle catalyst used in polymer electrolyte membrane (PEM) fuel cells. The cyclic voltammetric determination of hydrogen under potential deposition (Hupd) and carbon monoxide monolayer oxidation (CO stripping) performed in a fuel cell at fuel cell relevant conditions are compared and the influences of operation temperature (25-80 {degree sign}C) and relative humidity (RH) (40-90% RH) are discussed. The results show that both the shape and the charge of the Hupd are strongly dependent on operating conditions. However, for the CO stripping experiments only a negative shift in potential with increasing temperature or humidity was observed in accordance with results from aqueous electrolytes, whereas the charges for CO monolayer oxidation were almost constant for the temperatures investigated.