XPS and UPS have been used for studying the oxidation of TaSi₂ films formed by annealing, under ultrahigh-vacuum, of Ta deposits (≈70Å) on polycrystalline Si. Oxidation has been carried out at room-temperature and at 700°C with an oxygen pressure of 2×10^-5 mbar. Both Ta and Si react with oxygen. At room-temperature, O chemisorbs on the surface giving a mixed phase of Si suboxides and Ta oxides (TaO, TaO₂ and Ta₂O₅). At 700°C, a very thin Ta₂O₅ layer (<0.1 monolayer) is formed on top of the growing SiO₂ layer. In comparison with Si(111), the oxidation kinetics of TaSi₂ begins with a higher transitory rate and follows afterwards the same parabolic dependence of the SiO₂ thickness as a function of oxidation time. UPS spectra indicate a higher density of interface states, in comparison with that of SiO₂ on Si. The XPS valence band testifies that the electronic structure of the underlying TaSi₂ is preserved after oxidation. Subsequent heating of the sample leads to the progressive decomposition of SiO₂, at T 750°C, while the upper Ta₂O₅ layer is not affected. The results are discussed in relation to thermodynamic properties of Si and Ta oxides, and compared with existing results given by macroscopic methods.