Different MoO3/alpha-Al2O3 catalyst samples have been prepared using two preparation methods for a MoO3 deposit, the first one consisting of aqueous impregnation with sodium molybdate and the second one starting from MoO3 by an equilibrium adsorption in slurry MoO3/water (called the SIM method: slurry impregnation or solvent assisted spreading method). The alpha-alumina support was obtained by heating gamma-alumina at 1150 degrees C. gamma-alumina partially transformed to alpha-alumina by heating is used as catalyst support. For each kind of preparation two catalyst samples with different MoO3 loadings were prepared: 0.08 (around 0.04 equivalent monolayer) and 5.4 wt% (around 2.5 equivalent monolayers) and 1.2 (around 0.6 equivalent monolayer) and 5.2 wt% (around 2.4 equivalent monolayer) MoO3, respectively. They were characterized by X-ray diffraction, temperature programmed reduction, and X-ray photoelectron spectroscopy (XPS) measurements. In situ XPS measurements as a function of reduction time and temperature were performed and compared on the 5.4% MoO3/alpha-Al2O3 and 1.2% MoO3/alpha-Al2O3 catalyst samples. The original oxidic form of each catalyst sample, before any reduction treatment, shows only the presence of Mo(VI) surface species. For high molybdenum concentrations, the complete disappearance of Mo(VI) into Mo(V) and Mo(IV) has been observed after treatment for 9 h under hydrogen at 350 degrees C. After reduction for 5 h at 450 degrees C, Mo(0) appeared. For concentrations close to the monolayer, treatments for 10 h under hydrogen at 450 degrees C are necessary to reduce completely Mo(VI) into Mo(V) and Mo(IV); in that case, even a reduction at 700 degrees C is insufficient to observe Mo(0) and only Mo(IV) and Mo(II) surface species are detected. Thus, increasing MoO3 loading results in an increase of the catalyst reducibility. No Al-2(MoO4)(3) seems to be present before and after reduction steps. Interaction of molybdenum oxide with alumina is discussed, depending on the molybdenum oxidic contents and on the preparation of the catalysts.