In proton exchange membrane fuel cells, catalyst layers are key components for achieving higher performance and lower cost. Controlling the structure of the catalyst layers is important, since it can contribute to improving the transport of reactants and products in the catalyst layers, which in turn significantly affects overall potential loss. In this study, we adopted a method of controlling the structure of the catalyst layers by blending platinum-supported and stand-alone carbon black. Catalyst layers in which either thickness or platinum loading were controlled, keeping the other variable constant, were synthesized and evaluated in actual fuel cells. The results showed that lowering the platinum loading caused a drop in cell performance, while thinning the catalyst layer gave better performance. From analyses of the performance evaluations in various conditions, it was indicated that local mass transport (proton and/or oxygen) and water flooding in the catalyst layers plays an important role in cell performance under demanding operating conditions. (C) 2013 Elsevier B.V. All rights reserved.