(Ti, Al)N films have drawn much attention as alternatives for TiN coatings, which are oxidized easily in air above 500 degrees C. We have investigated the effect of Al content on the oxidation resistance of (Ti1-xAlx)N films prepared by r.f. reactive sputtering. (Ti1-xAlx)N films (0 less than or equal to x less than or equal to 0.55) were deposited onto fused quartz substrates by r.f. reactive sputtering. Composite targets with five kinds of Al-to-Ti area ratio were used. The sputtering gas was Ar (purity, 5 N) and N-2 (5 N). The flow rate of Ar and N-2 gas was kept constant at 0.8 and 1.2 sccm, respectively, resulting in a sputtering pressure of 0.4 Pa. The r.f. power was 300 W for all experiments. Substrates were not intentionally heated during deposition. The deposited films (thickness, 300 nm) were annealed in air at 600 similar to 900 degrees C and then subjected to X-ray diffractometer and Auger depth profiling. The as-deposited (Ti1-xAlx)N films had the same crystal structure as TiN (NaCl type). Al atoms seemed to substitute for Ti in lattice sites. The preferential orientation of the films changed with the Al content of the film, x. Oxide layers of the films grew during annealing and became thicker as the annealing temperature increased. The thickness of the oxide layer grown on the film surface decreased with increasing Al content in the film. For high Al content films an Al-rich oxide layer was grown on the surface, which seemed to prevent further oxidation. All of the films, however, were oxidized by 900 degrees C annealing, even if the Al content was increased up to 0.55.