The abatement of HCHO and CO at room temperature by catalytic oxidation over metal oxides can be achieved. However, the catalytic performance of metal oxides is influenced by H2O vapor in the reaction air and the mechanism of H2O role is still debated. In this paper, the effect of H2O on the catalytic performance of an MCo2O4 = (Mn, Ce and Cu) catalyst for HCHO and CO oxidation at room temperature was investigated. For both HCHO oxidation and CO oxidation, all the MCo2O4 catalysts in humid air are less active than in dry air except for HCHO oxidation catalyzed by CuCo2O4 catalyst. The CuCo2O4 is inactive for HCHO oxidation in dry air, but active for HCHO oxidation in humid air. XRD and XPS results indicated that there are no obvious changes of crystal structure and valance states of element between fresh and used MCo2O4 catalysts. The active sites of MCo2O4 catalysts and reaction/deactivation mechanism of HCHO and CO oxidation were studied by H2O-TPD and DRIFT. The associatively adsorbed H2O on MCo2O4 contributed to the active HOH sites for HCHO oxidation. Formate species are intermediates of HCHO oxidation. However, if formate cannot be further transferred into CO2, the formate adsorbed on active sites will result in catalyst deactivation. In humid air, hydrogen-bonded OH and free OH generated and covered active sites accelerated the catalyst deactivation in HCHO oxidation. The CO was oxidized by the active oxygen species but not included OH species. Thus, the H2O adsorbed on oxygen vacancies blocked the generation of active oxygen species, which resulted in MCo2O4 deactivation for CO oxidation in humid air. This work provide a fundamental understanding of the key role of H2O in the HCHO and CO oxidation at room temperature, which is helpful for the design of high activity and long lifetime catalyst used in dry or humid conditions.