Polycrystalline CdO:In 2 O 3 thin films for gas sensor applications were prepared on glass and silicon substrates by using one-step spray pyrolysis technique from the aqueous solution of CdCl 2 and InCl 3 at a substrate temperature of 300 °C. The structure, surface morphology, and the optoelectronic properties of prepared films were characterized respectively by means of X-ray diffraction (XRD), atomic force microscope and UV-visible spectroscopy. Based on the XRD results, the polycrystalline nature of CdO films has been confirmed, and In 2 O 3 films were found to exhibit a preferred orientation along (222) diffracted plane. The grain size varies between 9.0 and 28.4 nm. The results of Hall effect measurement of CdO:In 2 O 3 thin films confirms that all films were an n-type semiconductor. The electrical properties of prepared thin films and their sensitivity to nitrogen dioxide (NO 2) gas are also studied. The influence of the operating temperature and In 2 O 3 concentration on the NO 2 response were investigated. It is found that all films are sensitive to NO 2 gas, and the ideal operating temperature for the film contented 20 vol% of In 2 O 3 was found to be 200 °C at a gas concentration of 25 ppm. The sensing mechanism of the CdO:In 2 O 3 thin film is discussed and attributed to electron transfer between the sensing element and NO 2 molecules.