The effect of Al8B4C7 used as an antioxidant in MgO-C refractories and the behavior of Al8B4C7 in CO gas were investigated in the present study, Al8B4C7 was found to react with CO gas, to form Al2O3(s), B2O3(l), and C(s), at temperatures >1100 degreesC, The Al2O3 reacts with MgO to form MgAl2O4 near the surface of the material. lit the same time, B2O3(l) evaporates and reacts with MgO, to form a liquid phase, at >1333 degreesC, the eutectic point between 3MgO .B2O3 and MgO. The coexistence of the liquid and MgAl2O4 makes the protective layer mole dense, thus inhibiting oxidation of the refractory. At >1333 degreesC, the process apparently is controlled by oxygen diffusion, whereas it is controlled by chemical reaction when the temperature is <1333C.