Semiconductor sensors based on nanocrystalline In,O-2(3), thin films, which were prepared by a sol-gel method, showed high sensitivity to NO2. In2O3, sensing layers were obtained by calcination of Ln(OH), sol which was stabilized with nitric acid. These films were investigated in respect to their non-stoichiometry. The effect of annealing in air and in reducing atmosphere was studied by X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR). In XPS core level spectra taken upon heating in vacuo (620 K) an increase in the full width at half-maximum (FWHM) values of the In 3d doublet and a decrease in the oxygen to indium ratio (n(O),/n(In),) is found. ESR measurements show the presence of electronic and 'hole' centers (F-centers, In2+) in the sol-gel In-2,O-3, used for the preparation of the sensing layers. Their concentration is increased markedly by heating in vacuum (620 K) and in H-2 (470 K, H-2 under flow conditions). So the combined ESR and XPS investigation suggests the formation of non-stoichiometric In2O3-x,, upon annealing of stabilized sols and gives evidence for the presence of electronic and hole centers in the sensitive layers.