Stoichiometry of molybdenum oxide, MoOx, thin films was controlled by simply modulating the argon working pressure during the non-reactive sputtering deposition. Rutherford Backscattering Spectrometry revealed that the O/Mo ratio increased between 2.6 and 3.0 as the argon pressure increased. X-Ray Photoemission Spectroscopy (XPS) analyses pointed out that the sub-stoichiometry led to the formation of oxygen vacancies, closely related to the presence of Mo5+ reduced oxidation state, while in fully stoichiometric films only Mo6+ oxidation state appeared. The strong correlation between composition and optical properties was demonstrated through optical absorption measurements. Sub-stoichiometric films exhibited a narrower bandgap and significant infrared absorption at 1.55 eV, attributed to increased disorder and a higher number of defects, such as oxygen vacancies. Finally, by merging the XPS and …