The application of Al or Si as antioxidants in MgO-C refractories leads to the in situ formation of new phases. Due to uncertainties regarding phase formation in some references four different test series were heat treated from 800 to 1600 °C under reducing conditions in 200 °C steps. The antioxidant systems investigated were pure Al, Al and Si in two different ratios and SiC. The addition of SiC did not lead to the formation of new phases but in case of Al and Si addition carbides, nitrides, oxycarbonitrides and oxynitrides were formed. The reaction series, its reason and its relation to equilibrium phase composition have been revealed.