Data are presented to quantify the effect of reformate components on the performance of proton exchange membrane fuel cells (PEMFCs) with Pt and Pt/Ru alloy anodes. The performance deviates by 10-70 mV from Nernst behavior at current densities above 0.3 A/cm(2) when the cell is operated with N-2 /H-2 mixtures. The same deviations were observed with CO2/H-2 mixtures for a PEMFC with a Pt/Ru anode. However, for a PEMFC with a Pt anode, CO2-diluted H-2 gives significantly larger polarizations than N-2-diluted H-2. Also, cyclic voltammetry indicates that, after the anode is exposed to the CO2/H-2 mixture, some CO is produced through the reverse water gas shift (RWGS) reaction. These data are consistent with equilibrium calculations showing that the CO concentration can reach between 10 to 170 ppm. Consistent with CO production from CO2, the CO stripping areas (which are a measure of CO coverage! also showed dependence on the inlet composition of the anode gas as well as the presence of O-2 on the cathode side. The CO coverage resulting from the RWGS reaction approaches 5 x 10(-7) mol/cm(2) for a 0.4 mg/cm(2) Pt anode. (C) 2004 The Electrochemical Society.