The oxidation reaction of CO with O-2 on the FeOx/Pt/TiO2 catalyst is markedly enhanced by H-2 and/or H2O, but no such enhancement occurs on the Pt/TiO2 catalyst. Isotope effects were studied by H-2/D-2 and H2O/D2O on the FeOx/Pt/TiO2 catalyst, and almost the same magnitude of isotope effect of ca. 1.4 was observed for the enhancement of the CO conversion by H-2/D-2 as well as by H2O/D2O at 60 degrees C. This result suggests that the oxidation of CO with O-2 via such intermediates as formate or bicarbonate in the presence of H2O, in which H2O or D2O acts as a molecular catalyst to promote the oxidation of CO as described below.