Soft X-ray emission spectroscopy (SXES) technique combined with an electron probe micro analyzer is used to study the oxygen concentrations of the alpha-Ti and gamma-TiAl phases in equilibrium with each other in the Ti-Al-O ternary system. A series of alloys with various oxygen concentrations were prepared for standard and quantitative analysis. In order to optimize the analysis condition, the oxygen-K alpha peak intensities were measured under various probe currents, acceleration voltages, and dwell times for homogenized alpha single-phase samples with known oxygen concentrations after removing the oxygen-enriched layer on the surface by Ar sputtering. The calibration curve obtained under the optimized conditions shows excellent sensitivity to dissolved oxygen concentration as well as high linearity and an accuracy of better than 0.01 wt%. The combination of oxygen analysis using SXES and metal element analysis by wavelength dispersive spectroscopy revealed that the oxygen is enriched in the alpha phase and the Al concentration of the alpha phase in equilibrium with the gamma phase decreases. In the meantime, the gamma phase can dissolve only about 0.2 at% oxygen, and the alpha + gamma/gamma phase boundary moves significantly toward higher Al concentration with increasing oxygen in the alloy. These results suggest that interstitial oxygen prefers the Ti-O bonds in close-packed structures and can hardly have the Al-O bonds. (C) 2022 Elsevier B.V. All rights reserved.