The oxidation of methanol has been measured on MoO3 and MoO2. The properties of these two materials are interchangeable, depending upon the conditions in which the reaction is run. MoO3 produces high yields of formaldehyde, but MoO2 does not, due to the importance of the Mo6+ state for the selective reaction. However, if the MoO3 material is run in anaerobic conditions it behaves in a very similar way to MoO2, due to the presence of Mo4+ in the surface layers. In complement to this MoO2 converts to high yield behaviour when run in aerobic conditions, due to the conversion of the material to Mo6+ at the surface, and, ultimately to MoO3 in the bulk. In TPD experiments MoO3 yields formaldehyde, whereas MoO2 yields CO. In both materials oxygen transport within the lattice becomes appreciable above 300 degrees C, and the reaction proceeds via the Mars-van Krevelen mechanism.