The presence of CO in the H-2-rich gas used as fuel for hydrogen fuel cells has a detrimental effect on PEMFC performance and durability at conventional operating conditions. This paper reports on an investigation of the effect of CO on H-2 activation on a fuel cell Pt/C catalyst close to typical PEMFC operating conditions using H-2-D-2 exchange as a probe reaction and to measure hydrogen surface coverage. While normally limited by equilibrium in the absence of impurities on Pt at typical fuel cell operating temperatures, the presence of ppm concentrations of CO increased the apparent activation energy (E-a) of H-2-D-2 exchange reaction (representing H-2 activation) from approximately 4.5-5.3 kcal mole(-1) (Bernasek and Somorjai (1975)[24], Montano et al. (2006) [25]) (in the absence of CO) to 19.3-19.7 kcal mole(-1) (in the presence of 10-70 ppm CO), similar to those reported by Montano et al. (2006) [25]. Calculations based on measurements indicate a CO surface coverage of approximately 0.55 ML at 80 degrees C in H-2 with 70 ppm CO, which coincide very well with surface science results reported by Longwitz et al. (2004) [5]. In addition, surface coverages of hydrogen in the presence of CO suggest a limiting effect on hydrogen spillover by CO. Regeneration of Pt/C at 80 degrees C in H-2 after CO exposure showed only a partial recovery of Pt sites. However, enough CO-free Pt sites existed to easily achieve equilibrium conversion for H-2-D-2 exchange. This paper establishes the baseline and methodology for a series of future studies where the additional effects of Nafion and humidity will be investigated. (C) 2010 Elsevier B.V. All rights reserved.