We have measured the influence of adsorbed H2O on the sticking coefficients and saturation coverages of CO, O-2 and D-2 on Pt(111). at similar to 100 K. Strong poisoning is observed for all three gases. For O-2 and D-2, the surface is essentially totally poisoned at 1 monolayer (ML) water coverage. For CO, the effect is weaker, with some CO adsorption still occurring at 2-3 ML H2O. The influence of these results on the kinetics of the CO and H-2 oxidation reactions are discussed briefly It is concluded that the influence of water must be included in kinetics simulations, at least at low temperatures, when significant humidity levels are present in inlet gas mixtures, or produced by the reactions themselves.