The mechanism of the selective oxidation of methanol on Sn-Mo oxide catalyst with molar ratio Mo/(Mo + Sn) = 0.3 is investigated. The interaction of methanol and its oxidation products (i.e. formaldehyde and methyl formate) is studied by FT-IR spectroscopy. The data are interpreted on the basis of a reaction mechanism which contains 10 elementary steps. The behaviors of the surface species (methoxy, formate and dioxymethylene groups) are compared with those monitored on other systems and the catalyst requirements for the title reaction are discussed.