Thick film heterojunction gas sensors based on SnO2, WO3 and ZnO were fabricated. Their response to the presence of isopropyl alcohol was investigated. The sensing structures were studied using temperature stimulated conductance and cyclic voltammetry. The results of measurement shows, that the prepared structures exhibit current-voltage characteristics like a Schottky diode. This behavior result in strong influence of polarization voltage on the sensing parameters of those structures.