We report the formation of silicon oxide thin films obtained at room temperature by Ar+ bombardment of Si(100) wafers in partial oxygen atmosphere, The ion bombardment was done in the energy range E(b) - 100-1000 eV, with Ar partial pressure P-Ar = 1 x 10(-4) Torr, and O-2 pressure ranging in between 7 x 10(-7) and 7 x 10(-6) Torr. Samples have been investigated by core level X-ray Photoelectron Spectroscopy (XPS) using monochromatized Al K-a radiation with 0.28 eV total energy resolution. The formation of SiO2, in particular, was studied by Si2p core level spectra, Fitting of the experimental data has been done for a quantitative analysis of the SiO, thickness. Homogeneous SiO2 top layers up to 50 Angstrom thick were obtained. The role of the ion beam energy, O-2 pressure and irradiation time is investigated.