SiO x thin films have been prepared by evaporation of silicon monoxide powder in an ultrahigh vacuum chamber. The films are characterized by x-ray photoelectron spectroscopy (XPS), synchrotron photoemission, and x-ray absorption spectroscopy at the Si K edge. XPS shows that the films prepared by evaporation in ultrahigh vacuum have a SiO1.3 stoichiometry and are formed by Si3+(∼77%) and Si+(∼23%) species. Based on extended x-ray absorption fine structure analysis, the structure of these films has been described as formed by tetrahedra of the type Si–(Si, O3) and Si–(Si3, O), in agreement with the Si 2p photoelectron spectra. No significant amount of Si2+ species [i.e., Si–(Si2, O2)] tetrahedra) or elemental silicon were detected in these films. When SiOx thin films are prepared by evaporation of silicon monoxide in O2 atmosphere, the oxygen content in the film increases with the partial pressure of this gas. Under these conditions, Si4+ species are formed in detriment of the Si+ and Si3+ oxidation states. The relative concentration of the different oxidation states of silicon is discussed in connection with previous models existing in literature on the distribution of Sin+ states in SiOx thin films. An explanation based on molecular orbital calculations is proposed to justify the absence of Si2+ species in the film structure and the disproportionation reaction Si3+, (Si+)→Si4+, Si0 observed when the SiOx thin films are excited by exposure to the white light of the synchrotron radiation.