The oxidation of methanol has been studied on Mo/Al2O3 catalyst in absence and presence of oxygen. Quasi-simultaneous IR and Raman spectroscopies were used to follow the evolution of the active phase and adsorbed species on the catalyst under reaction conditions. An increase of reduced Mo species on the active phase is observed by Raman spectroscopy in parallel with a deactivation of the redox properties of the catalyst. Complementary EPR experiments show the transformation of Mo-VI to Mo-V and its accumulation in absence of oxygen. Selectivity toward oxidation products is drastically affected by the accumulation of Mo-V species on the active phase. Oxygen addition to the reaction mixture leads to a decrease of the amount of Mo-V and a partial restoration of the oxidative properties of the catalyst. (c) 2005 Elsevier B.V. All rights reserved.