As a measure of catalytic activity, i(eta=0.3 V), the current density at 0.3 V overpotential, was chosen to evaluate the oxygen reduction reaction (ORR) at elevated temperatures (> 100 degrees C) and various relative humidities (RH) for polymer exchange membrane (PEM) fuel cells. The purely kinetic reaction order of the ORR with respect to oxygen partial pressure is less than 1.0 and changes with the RH. The activation energy is 49 kJ/mol at 100% RH and 55 kJ/mol at 50% RH. The active electrochemical surface area of platinum changes little with RH. RH has a strong effect on the catalytic activity under dry conditions(0-60% RH), but under wet conditions (> 60% RH) its influence is unclear. The Tafel slope obtained in the 1-100 mA/cm(2) current density range changes significantly with RH: wet conditions produce low Tafel slopes (< 80 mV/dec), dry conditions produce high Tafel slopes (> 100 mV/dec). Dependence of the RH on the oxygen reduction reaction (ORR) may be explained by the changes of the rate-determining reaction, proton activity, and adsorbed -OH on the platinum surface. The ORR kinetic parameters obtained here are instructive for high-temperature fuel cell data analysis and performance improvement. (c) 2005 The Electrochemical Society.