The thermochemical stability of the main refractory oxides and some of their compounds under strong reducing conditions has become increasingly important in recent years. In the present work, the formation, sintering and stability of magnesium-alumina spinel (MgAl2O4) under very low oxygen partial pressure were studied using Ellingham diagrams. Reduction processes of spinel at high temperatures 1500-1650degreesC were discussed. The experimental treatments were carried out with spinel and equimolar mixtures of MgO and Al2O3. The effect of the oxygen partial pressure on stability of spinel was evaluated using different atmospheres: air (P-O2 = 0.21 atm), CO/CO2 (80/20), (P-O2 < 10(-10) atm), CO (P-O2 < 10(-15) atm) and C-CO (P-O2 < 10(-20) atm). Finally, the results were compared with the experimental observations of spinel formation in MgO-C refractories with the addition of aluminium. (C) 2004 Elsevier B.V. All rights reserved.