Abstract: Platinum-based catalysts are considered the most efficient catalysts for triggering electrochemical reactions in proton exchange membrane (PEM) fuel cells. In the present study, commercial catalysts containing 10% and 30% Pt supported on Vulcan XC-72 carbon were studied to assess their performance in PEM fuel cells. Both catalysts consisted of Pt particles of almost the same size. The utility of these catalysts in PEM fuel cells was studied by finding the real surface area and rate of electro-oxidation of methanol in 0.5 M H₂SO₄ by using cyclic voltammetry. The methanol oxidation reaction was used for characterization of catalysts of PEM fuel cells due to the liquid nature of methanol and the close resemblance of basic electrochemical features of direct methanol fuel cells and PEM fuel cells. Comparison of the data of real surface area and rate of electro-oxidation of methanol showed that 30% Pt catalyst having higher Pt loading is more suitable for PEM fuel cells as compared with 10% Pt catalyst. The PEM fuel cell components were designed and fabricated for testing of membrane electrode assemblies (MEAs). The importance of an additional gas diffusion layer in the form of carbon paper was also emphasized for improving gas diffusion and electrical contact of electrodes of MEAs with the flow field area of monopolar/bipolar plates of PEM fuel cells. The MEAs prepared from 10% and 30% Pt catalysts with Pt loading of 0.5 mg cm^-2 gave the maximum power density of 119 and 185 mW cm^-2, respectively. It was concluded that nanosize carbon supported Pt catalysts having higher Pt loading are more suitable catalysts for preparing high performance MEAs of PEM fuel cells and the cyclic voltammetric data of real surface area and rate of methanol oxidation may be utilized to assess the performance of a given catalyst prior to its use in the preparation of MEAs of PEM fuel cells.