Adsorption　of　H2O　and　its　effect　on　CO　oxidation　over　spinel　Co3O4　(110)　surface　were　studied　by　density　functional　theory　calculations.　H2O　is　adsorbed　favorably　at　the　octahedral　cobalt　(Cooct)　site　through　O　atom　on　the　surface.　Hydrogen　bonding　interaction　between　1s　orbitals　of　H　atoms　in　H2O　and　the　2p　orbitals　of　surface　active　oxygen　sites　plays　a　key　role　for　H2O　adsorption.　The　inhibition　effect　of　H2O　adsorption　on　the　CO　oxidation　over　the　surfaces　is　attributed　to　the　competition　between　H2O　and　CO　molecules　for　the　surface　twofold　coordinated　oxygen　site.　©　2011　Elsevier　B.V.　All　rights　reserved.