Operation of polymer electrolyte membrane fuel cells at elevated temperatures and low relative humidity is limited by the electrolyte's dependence on water. The addition of a heteropolyacid, specifically silicotungstic acid (STA), is shown to increase the activity of platinum for the oxygen reduction reaction by nearly 73% at 118 degrees C/40% relative humidity. However, STA-containing electrodes have a significantly higher diffusion resistance, countering any activity improvement at higher currents. By reducing the electrode ionomer weight fraction, the diffusion resistance decreases. Also, membrane durability is improved significantly by including STA at the cathode. (C) 2009 The Electrochemical Society. [DOI:10.1149/1.3216001] All rights reserved.