Addition of magnesium cations to gamma (and delta) alumina enhances its rate of transformation into alpha alumina, whereas addition of zirconium cations inhibits it. The effect of adding both magnesium and zirconium cations at various concentrations from 0.5 to 5% to aluminium ions has been studied. The kinetic curves of transformation into alpha alumina at 1373 K are reported. The effects observed are in agreement with the D.T.A. measurements. Changes in surface area accompanying the phase transformation have also been measured. A synergy effect may occur between magnesium and zirconium behaviours, since maximum stabilization of the surface area is found with a particular co-doped product (Mg²⁺(1%)-Zrsup>4+(0.5%)). In the case of pure alumina, the influence of water vapour in the range 1-20 torr has also been investigated with the same methods. The results show that water vapour enhances the rate of transformation into alpha.