Using thin-film technology, MOS structures Al/WO3/Si and Al/SnO2/Si were obtained by vacuum condensation of the oxides and the metal. V-I and high frequency C-V curves of these structures are characteristic of heterodiodes. Their light irradiation results in significant photovoltaic effect. The remarkable advantage of such photodiodes is the possibility of direct light irradiation of large-area heterojunction through transparent oxide layers. The gas sensitivity of these metal oxides has been studied in a view of the development of semiconductor sensors for determining the concentrations of H-2, O-2, and ethanol vapor constituents in the air. The response times and operating temperatures of the sensors have been obtained. Physical principles of gas detectors design are discussed.