Gallium oxide thin films were prepared by thermal evaporation and deposition of Ga2O3 on NaCl(001) cleavage planes at varying substrate temperatures, oxygen pressures and deposition rates. The structure of the so-prepared thin films was checked by Transmission Electron Microscopy and Selected Area Diffraction and also characterized by X-ray Photoelectron Spectroscopy and Atomic Force Microscopy, both in the as-deposited state and after different oxidative and reductive treatments. The substrate temperature proved to be most crucial for the structure of the gallium oxide films, ranging from low-contrast amorphous structures at low substrate temperatures (298 K) to nanosphere structures at higher temperatures (580 K). The stability of the films was found to be mainly determined by the interaction of substrate temperature and deposition rate. Crystalline beta-Ga2O3 structures were obtained after oxidative, reductive and annealing treatments at and beyond 773 K suggesting that the crystallization is mainly a thermal annealing effect. (C) 2007 Elsevier B.V. All rights reserved.