The effect of Nafion loading in the cathode catalyst layer of proton exchange membrane fuel cell (PEMFC) electrodes was studied by impedance spectroscopy, cyclic voltammetry, and polarization experiments. Catalyst utilization, determined by cyclic voltammetry, peaked at 76% for a Nafion loading of ca. 30 mass %, and this coincides with the optimum performance obtained in H-2/O-2 fuel cells. However, the small range of utilizations observed (55-76%) cannot explain the wide range of performances. The impedance results show that the ionic conductivity of the cathode increased greatly with increasing Nafion content, and this is the main factor responsible for the increase in performance up to 30% Nafion. The loss of performance at higher Nafion loadings must have been due to an increasing oxygen transport resistance, because the electronic resistance did not increase significantly. In fact, the highest electronic resistances were observed at low Nafion loadings, indicating that Nafion played a significant role as a binder. (C) 2003 The Electrochemical Society. All rights reserved.