Mixed Ti _xSi _{1 − x}O ₂ oxide can exhibit a partial phase separation of the TiO ₂ and SiO ₂ phases at the atomic level. The quantification of TiO ₂–SiO ₂ mixing in the amorphous material is complicated and was so far done mostly by infrared spectroscopy. We developed a new approach to the fitting of X-ray photoelectron spectroscopy data for the quantification of partial phase separation in amorphous Ti _xSi _{1 − x}O ₂ thin films deposited by plasma enhanced chemical vapour deposition. Several fitting constraints reducing the total number of degrees of freedom in the fits and thus the fit uncertainty were obtained by using core electron binding energies predicted by density functional theory calculations on Ti _xSi _{1 − x}O ₂ amorphous supercells. Consequently, a decomposition of the O 1s peak into TiO ₂, SiO ₂ and mixed components was possible. The component areas ratios were compared with the ratios predicted by older theoretical models based on the atomic environment statistics and we also developed several new models corresponding to more realistic atomic structure and partial mixing. Based on the comparison we conclude that the studied films are mostly disordered, with only a moderate phase separation.