MgO-graphite refractory bricks have been fabricated with 0-6% antioxidizing additions of Si and Al. The phases and microstructures resulting from heat treatments in CO/N-2 atmosphere at 1173 and 1873 K have been fully characterized using X-ray diffraction, polarized light, and scanning electron microscopy techniques. The metal additions react to form a variety of phases and preferentially oxidize protecting the carbon in the microstructure. Calculations performed with MTDATA and F*A*C*T were used to predict the phases Likely to be present. AU of the observed phases were predicted, and the dependence of the occurrence of the phases on temperature and position in the composite enabled a mechanism to be proposed for the formation of the principal features of the microstructure.