Reformed gas containing Co-2, N-2, and H-2 may be used in proton exchange membrane fuel cells (PEMFCs), and recent evidence has shown that CO2 can react in situ with H-2 (i.e., a reverse water gas shift (RWGS) reaction) and produce adsorbed CO that can poison the electrode catalyst. Here, a study is presented to extend the previous observations by considering how pressure, gas composition, and temperature affect this reaction in a PEMFC for both Pt and Pt/Ru alloy catalysts. The coverage of CO produced on the electrodes was determined by stripping cyclic voltammetry (CV). The data show how the CO stripping potential depends on temperature, and how the analysis allows the determination of an activation energy. The data are shown to be consistent with a kinetic catalytic model and not with an equilibrium model. (C) 2004 Elsevier B.V. All rights reserved.