SiOx 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+(similar to 77%) and Si+(similar to 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, O-3) and Si-(Si-3,O), in agreement with the Si 2p photoelectron spectra. No significant amount of Si2+ species [i.e., Si-(Si-2, O-2)] tetrahedra) or elemental silicon were detected in these films. When SiOx thin films are prepared by evaporation of silicon monoxide in O-2 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+, Si-0 observed when the SiOx thin films are excited by exposure to the white light of the synchrotron radiation.