The influence of hot-pressing temperature on catalytic activity and the performance of proton exchange membrane fuel cells (PEMFCs) was investigated using current-voltage (I-V) polarization, electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). EIS provided detailed information on the contribution, from high to low frequencies, of internal impedance (R-s), interfacial impedance (R-if) and reaction impedance (R-rxn). The ohmic resistance of the cell (R-Omega) was estimated from the I-V diagram for comparison with the R-s and R-if impedances. The Rif is useful for diagnosing catalytic activity and interpreting PEMFC performance. A cell was hot-pressed at 125 degrees C (near the transition temperature of Nafion), an optimum temperature for lowest ohmic resistance and total impedance in response to a maximal catalyst-specific activity.