In2O3-x thin films with a thickness of 100-990 nm were grown by dc magnetron sputtering. Their structural, electrical and ozone sensing properties were analyzed. Structural investigations carried out by electron probe micro analysis, secondary ion mass spectrometry and atomic force microscopy showed a strong correlation between stoichiometry, surface topology and gas sensitivity. Moreover, the electrical conductivity of In2O3-x thin films exhibited a change of over six orders of magnitude during photoreduction with ultraviolet light and subsequent oxidation in ozone atmosphere at room temperature.