We prepared (Ti1-xAlx)N thin films by dc reactive sputtering using alloy targets and investigated their oxidation properties. The oxidation rate of the (Ti1-xAlx)N film decreases with increasing aluminum content because Al2O3, with a low oxygen diffusion coefficient, is segregated at the film surface. In particular, the phase separation into a double layer of Al2O3 and TiO2 is more noticeable at high temperature. On the other hand, at an initial stage of oxidation, a mixed oxide of titanium and aluminum (> 5 nm thick) is formed after oxidation at 550 degreesC for 1 min. This mixed-oxide formation is a result of insufficient growth of the Al2O3 layer, which causes fast oxygen diffusion and hence high oxidation rate. We thus conclude that (Ti1-xAlx)N is difficult to use as a barrier metal for metal-insulator-metal capacitors as long as the oxidation resistance is based on the growth of a protective Al2O3 layer on the film surface.