The mass and specific activities are intensively used as the characterization parameters for the effectiveness of the catalyst in a fuel cell. These quantities were found to be independent of catalyst loading, but the data presented in this work shows that the mass and specific activity are not independent of the loading at ultra-low Pt loadings. In this work the dependence of mass activity on platinum loading was studied. A one dimensional simplified agglomerate-based macro-homogeneous model was developed and solved analytically and numerically to understand the effects of porosity, agglomerate size, and Pt loading on the mass activity.