Nanosized alumina powders were synthesized by mechanochemical treatment of stoichiometric mixture of anhydrous AlCl3 and CaO. X-ray powder diffraction (XRD), differential thermal and thermogravimetric analysis (DSC-TG), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to understand the formation mechanism and the exact phase transformation sequence in this system. Milling of the initial raw materials resulted in the formation of crystalline CaO and partially amorphous AlCl3, with no sign of chemical reaction between the constituents. Heating the as-milled powder led to the formation of amorphous aluminum hydroxide (Al(OH)(3)) and calcium chloride (CaCl2). Based on the results obtained in this study, AlCl3 hydrolyses in the heating stage and CaO adsorbs the produced HCl forming CaClOH and CaCl2 crystalline phases. Heat treatment of the amorphous Al(OH)(3) resulted in the formation of amorphous alumina (Al2O3). Amorphous alumina transformed into eta-, kappa-, and alpha-Al2O3 when calcined at higher temperatures. Based on Rietveld refinement, it was concluded that eta-Al2O3 and kappa-Al2O3 phases are stable up to an average critical crystallite size of around 13 nm and 39 nm respectively. (C) 2012 Elsevier B.V. All rights reserved.