The interaction of evaporated Co atoms with polycrystalline MoO3 surfaces at 90 and 300 K was studied by means of X-ray photoelectron spectroscopy (XPS). The MoO3 samples (similar to 300 nm thick) were prepared by oxidation of a Mo 99.9% foil at 800 K in a flow of O-2 at similar to1 bar. X-ray diffraction spectra indicated that only MoO3 was obtained. From the very beginning of the deposition the Co 2p spectra showed the presence of Co2+ species even at 90 K, while at higher coverages the Co-0 increased steadily. Simultaneously, a shoulder in the Mo 3d spectra indicated the formation of Mo4+ and Mo5+ species. The following annealing up to 800 K revealed that above 500 K, the Co-0 species also begin to be oxidized. A careful deconvolution procedure of the experimental signal indicates that part of the Mo4+ and Mo5+ species transformed slowly into Mo6+. At both deposition temperatures, after a thorough annealing at 800 K the main product obtained was a non-stoichiometric Co-molybdate, as data of the binding energies of the Mo 3d peaks of this compound suggest.