In the present study we examine the changes in phase composition and microstructure undergone by solids after thermal treatments performed on transition aluminas in chlorine and air atmospheres. The thermal transformation to the stable alpha-Al2O3 was faster for the chlorine-heated samples than for the air-heated ones, and for chlorinated samples single crystals of alpha-Al2O3 were observed. We propose a mechanism by which AlCl3(g) formation allowed vapour mass transport that contributed to the growth of crystals of alpha-Al2O3. Hydroxyls contained in transition aluminas could enhance gaseous transport and could also react with chlorine molecules. The subsequent alpha-Al2O3 crystallization would be assisted by water vapour.