X-ray photoelectron spectroscopy (XPS or ESCA) measurements were performed on a photoreduced Mo allyl-based Mo/SiO2 catalyst (1.7 wt % Mo). Photoreduction was carried out in CO at room temperature for increasing lengths of time. Factor analysis, deconvolution, and curve fitting were used to determine the number and positions of components present in the series of XPS Mo 3d spectra obtained for the oxidic and photoreduced Mo/SiO2 catalyst. The results indicated the formation of a single oxidation state Mo+ (Mo 3d(5/2) binding energy at 232.1 eV) after 2 h of photoreduction and the appearance of Mo4+ as in MoO2 (Mo 3d(5/2) binding energy at 230.2 eV) with continued photoreduction. Average oxidation state studies indicated that the species designated as Mo-x+ should also be assigned to Mo4+ but in an environment different than that of MoO2.