Exposures to oxygen at room temperature and annealings under vacuum were carried out on deposits obtained from molybdenum interacting with(1 1 0) TiO2 surfaces in order to obtain molybdenum oxide ultra thin films. Exposures to oxygen at room temperature show that the interfacial molybdenum oxide layers resulting from the TiO2/Mo interactions are inactive towards oxygen whereas the metallic molybdenum clusters, which grew on top of the interfacial layers, oxidise into MoO3. Besides, during annealings under vacuum, substrate oxygen anions can diffuse into the deposit. Thus, between 400 and 500 degreesC, molybdenum oxide layers are progressively oxidised into MoO2. Moreover. from the annealing temperature, it is possible to control the him oxidation and to select its stoichiometry. At higher temperatures, MoO2 layers oxidise. leading to MoO3 clusters which sublimate restoring a clean TiO2 surface. As concerns the metallic clusters, if they are exposed to air and then oxidised in MoO3 prior to annealing. they sublimate below 300 degreesC.