The structure, phase composition and element distribution of the oxidation scales were analyzed by XPS for the Ti-42Al-8Nb alloys which was oxided at 900 oC for 20 h in air. It is noted that the outside of oxidation scale was composed of TiO₂ only achieved by XPS, which was different from the results of the XRD analysis. The mixture of Al₂O₃, three titanium oxides and a few nitrides exist near the outside of the scale. The contents of Ti oxide with high valence state increased along the direction from the substrate to the outside of the sample. There was no presence of the other two Ti oxides with lower valence state in the outside of the scale, only TiO₂ existed. It is concluded that the whole process of the TiAl alloy was promoted by the dissolution, diffusion of the O atom, which resulted in the multi-stage variation in the scale with different depth for different oxidation time, meanwhile resulted in the differences of the structure, composition and contents in the oxidation scales.