The initial rate of hydrogen dissociation was studied as a function of irreversible CO coverage at 353 K on 30 wt.% Pt/carbon catalysts (Pt/C prepared according to different processes. The Pt/C catalysts exhibit similar Pt dispersion (D similar to 0.07) and mean Pt particles size (dp similar to 16 nm). The turnover frequency (number of hydrogen molecules dissociated per CO-free surface Pt atom) was determined as a function of CO coverage from 0.0 to 0.8. The evolution of TOF as a function of CO coverage is in agreement with the model of CO adsorbing on low coordination sites (edges, corners) and then spreading across the faces to grow islands as Brandt suggested in the past (R.K. Brandt, M.R. Hughes, LP. Bourget, K. Truszkowska, R.C. Greenlet, Surf. Sci. 286 (1993) 15-25). At high CO coverage (similar to 0.8), TOF depends on the process by which the Pt/C catalyst was prepared. In particular, a Pt/C elaborated according to a colloidal process exhibits a low sensitivity to CO poisoning with an increase of TOF by one order of magnitude. (C) 2009 Elsevier B.V. All rights reserved.