The structural properties of La2O3 and Al2O3-La2O3 binary oxides prepared by sol-gel were studied by XRD, HRTEM and UV-vis. The binary oxides with high lanthana contents show an amorphous structure after calcination at 650 degrees C. At calcination temperatures higher than 1000 degrees C there is a phase transformation from the amorphous state to the crystalline LaAlO3 with a perovskite structure. The structure of La2O3 is consistent with the hexagonal system; however, some crystalline microdomains with a monoclinic structure were detected by HRTEM. Islands of La2O3 and LaAl11O18 phases were detected at high lanthana concentration in the binary oxide. The modification in the coordination shell of the Al3+ cations due to the interaction with La3+ cations confirms the formation of phases with a perovskite structure and the presence of islands of the LaAl11O18 phase. (c) 2006 Elsevier Ltd. All rights reserved.