The high-temperature oxidation and the rate of corrosion by the V_2O_5 attack were examined with Ni-Cr-Mo, Ni-Cr-Co-Mo alloys containing about 19 per cent chromium and with some commercial heat-resisting alloys. In addition, the oxides composing thin oxide films or scales in these alloys were identified from observations of electron and X-ray diffractions. Following results were obtained. (1) Addition of molybdenum to Ni-Cr alloys increased the weight loss due to scaling above 1000℃, four sorts of oxides, Cr_2O_3, spinel phase NiOCr_2O_3, NiO and NiOMoO_3, being formed in the scales. Above 1200℃ NiOMoO_3 decomposed soon after the formation, with the volatilization of MoO_3, and thus the oxidation losses remarkably increased in Ni-Cr-Mo alloys. In these alloys, a rapid formation of oxides was observed at 1350℃, owing to the formation of molten phase of NiOMoO_3 in the scales. (2) A rapid oxidation occurred in Ni-Cr-10 Co-5 Mo alloy at 1300℃, owing to the formation of melt of (Ni, Co) OMoO_3 in the scale. The similar rapid oxidation was observed in molybdenum-containing Ni-base, Co-base and Fe-Ni-Co-Cr base heat-resisting alloys, which is considered to be associated with the formation and the fusion of (Ni, Co) OMoO_3, corresponding to nickel and cobalt content. (3) Addition of molybdenum increased the corrosion rate by V_2O_5 attack on Ni-Cr and Ni-Cr-Co alloys. Also the V_2O_5 attack was increased by molybdenum content in commercial heat-resisting alloys. Of the molybdenum-containing Fe-base, Co-base and Ni-base heat-resisting alloys, Ni-base alloys were the most resistant to the attack, Fe-base alloys being the least
