The mass activity is intensively used as characterization parameters for evaluation of the effectiveness of the cathode catalyst in polymer electrolyte membrane (PEM) fuel cells. In this work, the dependence of mass activity on platinum loading was studied at the cathode. The results indicated that the mass activity and the utilized electrochemical surface area of the catalyst are not independent of the catalyst loading. The electrochemical specific surface area (ECSA) and the mass activity increase as the loading is decreased. The increase of the ECSA is attributed to the increase of the utilization of the catalyst. The commonly applied Tafel-approximation cannot be used to fit the result because the mass activity is controlled not only by R-Omega and the ORR kinetics, but also the utilization of the catalyst, which in turn depends on catalyst loading, the structure of the catalyst layer, the degree of agglomeration, and screening of the catalyst particles. A detailed and more precise definition of mass activity (MA) is given to elucidate the variation of MA with catalyst loading. (C) 2013 Elsevier B.V. All rights reserved.