This paper concerns a study of the simultaneous addition to a mixture of gamma- and delta-alumina of Mg and Zr, which are known to accelerate and inhibit the transformation to theta- and alpha-alumina. The doped powders were calcined at 1105 °C and the alpha-phase content determined by XRD, and the surface area by the BET method. The best surface area stabilisation (60 m²/g) was obtained for alumina co-doped with 1% Mg and 0.5% Zr. The highest and lowest conversion to alpha-alumina were observed for a single additive, Mg and Zr respectively. Mg was found to enhance the theta-to-alpha transformation rate and to inhibit the formation of the theta-form from the gamma- and delta-phases. With Zr both the formation of theta and alpha were inhibited.