Powders with composition (AlxGa (1-x))(2)O-3 (x = 0, 0.25, 0.50, 0.75, 1) have been prepared by coprecipitation from aluminum nitrate and gallium nitrate and have been charaterized after drying at 373 K and calcination at 673, 1073 and 1473 K, using XRD, DTA, FT-IR and diffuse reflectance UV-VIS and BET surface area and porosity measurements. At 373 K Al compounds are essentially amorphous and for the Ga-0.75 Al-0.25 sample a solubility of 12% of Al3+ in the diaspore type α-GaOOH has been calculated. In samples calcined at 673 and 1073 K for the Al-rich ones a poorly crystalline defective spinel-type phase γ-Al2O3 has been found while the Ga-rich materials are composed of the metastable corundum type phase α-Ga2O3 and the thermodynamically β-Ga2O3 at 673 and 1073 K respectively. Al3+ added to α-Ga2O3 tends to hinder the α → β phase transition of gallium oxide. At 1473 K mixed oxides give rise the thermodynamically stable α-Al2O3 and β-Ga2O3 phases. Reciprocal solubilities of Ga and Al in their oxides have been calculated. (C) 2005 Springer Science + Business Media, Inc.