Experiments were performed to determine the carbon monoxide (CO) tolerance of an atmospheric pressure, hydrogen-oxygen, proton-exchange-membrane (PEM) fuel cell at elevated temperatures (80 to 120degreesC). A Nafion-Teflon-Zr(HPO4)(2) composite membrane was used to avoid the high resistance normally encountered with a Nafion membrane due to dehydration at the low reactant relative humidities encountered under these conditions. A Pt-Ru/C (40 wt % precious metal, Pt/Ru = 1/1 atomic ratio) anode catalyst of 0.4 mg Pt-Ru/cm(2) loading was used. The anode polarization decreased significantly as the cell temperature was increased from 80 to 105 to 120degreesC due to both smaller CO coverage on the anode catalyst and improved activity of the catalyst for CO and hydrogen oxidation. At elevated temperatures, the loss of water at the anode resulted in an increase in the measured cell resistance with increasing CO concentration. This effect is presumed to be due to an increase in the anode polarization that was partially measured along with the membrane resistance, when determined using the current interruption technique. The cell performance did not increase monotonically with temperature. When the CO concentration in the hydrogen at the anode was above 100 ppm, the fuel cell performance at 105degreesC was higher than that at 80degreesC. Dehydration of the membrane and catalyst layers tended to offset improved CO tolerance at elevated temperatures from 105 to 120degreesC at the same reactant water vapor content. (C) 2004 The Electrochemical Society.