Ultra-high vacuum thermal desorption experiments have been carried out on the adsorption of water, and the D2O/H and D216O/18O exchange reactions on platinum previously characterized by oxygen and hydrogen adsorption studies. Water is weakly adsorbed and only at T < 150 K, an observation confirmed by using H2 adsorption as a chemical probe for the presence of adsorbed water. The thickness of the adsorbed D2O layers was determined by a novel method involving monitoring the power output of the temperature programmer. Exchange occurs between adsorbed H and a D2O overlayer and the extent of exchange increases with the thickness of the overlayer. Isotope distribution studies show that the surface is heterogeneous and the exchange reaction does not occur at uniform rate. Exchange occurs between adsorbed 18O and a D216O overlayer. The extent of exchange is constant up to T ∼ 300 K showing that D2O is more strongly bound on O-covered Pt presumably as a result of H bonding. Heterogeneity also is apparent in the O-exchange reaction. The apparent activation energies for exchange are 1.8 and 2.9 kcal mol−1 for the half- and fully-covered surfaces, respectively.