△G^0-T diagrams of the reactions between some typical transition metal carbides （or borides） and oxygen were drawn using Matlab software. On the basis of these △G^0-T diagrams, equilibrium partial oxygen pressures were obtained accordingly. The volatility diagrams for ZrC-ZrO2 system and SiC-SiO2 system at 2500 K were drawn and the function of the partial press of various related products in the ZrC-ZrO2 system with temperature was evaluated. The calculation results indicated that the oxygen activity and equilibrium partial oxygen pressure of these carbides changed with temperature. According to the volatility diagrams of ZrC-ZrO2 and SiC-SiO2 systems, the vapour pressure of related oxides may be very high at the interfaces, even higher than 105 Pa, which is detrimental to the formation of entire oxide scales on carbides during ultra-high temperature oxidation. This work provided significant insight into the behaviour of ultra-high temperature oxidation of C/C composites and ultra-high temperature ceramics. It especially provided theoretical considerations for choosing oxidation-resistance dopants in C or C/C composites.
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