The effect of water vapor on the high-temperature oxidation behavior of commercial Ti-6Al-4V (Ti-6-4) and Ti-4.5Al-3V-2Mo-2Fe (SP-700) alloys was studied over the temperature range of 700~900 ℃ in air. The results showed that the oxidation kinetics of the two alloys followed the parabolic rate law, and the oxidation rate constants (kp values) of the alloys gradually increased with increasing temperature. It was found that kp values of the two alloys (Ti-6-4 and SP-700) oxidized in air containing water vapor were much faster than those in dry air. The scales formed on the alloys consisted mostly of TiO2 and minor Al2O3. The scale thickness of the two alloys in water-vapor containing air is much thicker and more porous, which is responsible for the fast oxidation rates, as compared to that in dry air.