Polarization losses of proton exchange membrane (PEM) fuel cells at 120 degrees C and reduced relative humidity (RH) were analyzed. Reduced RH affects membrane and electrode ionic resistance, catalytic activity and oxygen transport. For a cell made of Nafion (R) 112 membrane and electrodes that have 35 wt.% Nafion (R) and 0.3 mg/cm(2) platinum supported on carbon, membrane resistance at 20%RH was 0.407 Omega cm(2) and electrode resistance 0.203 Omega cm(2), significantly higher than 0.092 and 0.041 Omega cm(2) at 100%RH, respectively. In the kinetically controlled region, 20%RH resulted in 96 mV more cathode activation loss than 100%RH. Compared to 100%, 20%RH also produced significant oxygen transport loss across the ionomer film in the electrode, 105 mV at 600 mA/cm(2). The significant increase in polarization losses at elevated temperature and reduced RH indicates the extreme importance of designing electrodes for high temperature PEM fuel cells since membrane development has always taken most emphasis. (c) 2006 Elsevier Ltd. All rights reserved.