CO affects H-2 activation on supported Pt in the catalyst layers of a PEMFC and significantly degrades overall fuel cell performance. This paper establishes a more fundamental understanding of the effect of humidity on CO poisoning of Pt/C at typical fuel cell conditions (80 degrees C, 2 atm). In this work, direct measurements of hydrogen surface concentration on Pt/C were performed utilizing an H-2-D-2 switch with Ar purge (HDSAP). The presence of water vapor decreased the rate of CO adsorption on Pt, but had very little effect on the resulting CO surface coverage on Pts (theta(CO)) at steady-state. The steady-state theta(CO)s at 80 degrees C for Pt exposed to H-2 (P-H2 = 1 atm) and a mixture of H-2/H2O (1 atm H-2, 10%RH) were 0.70 and 0.66 ML, respectively. Furthermore, total strongly bound surface hydrogen measured after exposure to H-2/H2O was, surprisingly, the sum of the exchangeable surface hydrogen contributed by each component, even in the presence of CO. In the absence of any evidence for strong chemisorption of H2O on the carbon support with/without Pt, this additive nature and seemingly lack of interaction from the co-adsorption of H-2 and H2O on Pt may be explained by the repulsion of strongly adsorbed H2O to the stepped-terrace interface at high coverages of surface hydrogen. (C) 2011 Elsevier B.V. All rights reserved.