Amorphous gallium oxide thin films were prepared by the ultrasonic spray pyrolysis method using gallium acetylacetonate as source material and water as oxidizer. Samples annealed at 850 degreesC during 1 h show the crystalline beta -phase of Ga2O3. Rutherford backscattering results indicate that both as-deposited and annealed films have the stoichiometric chemical composition without incorporation of carbon impurities. Infrared (IR) spectroscopic measurements show that there is no incorporation of O-H and Ga-OH radicals in any of the studied films. The LR spectra for amorphous films show a broad absorption band from 400 to 900 cm(-1), typical for some amorphous metallic oxides. Meanwhile, for the annealed films the IR spectra show well-defined peaks located at 450 and 670 cm(-1) related to the beta -phase of Ga2O3. The refractive index of the films shows a strong change from 1.846 for the amorphous films to 1.935 for the annealed ones. The optical bandgap energy values are 4.94 eV for the as-deposited films and 4.99 eV for the annealed films. All these changes are associated with a different microstructure of the annealed films. (C) 2001 The Electrochemical Society. All rights reserved.