Tin oxide thin films were prepared by using a spray pyrolysis technique. It has been shown that the tin oxide thin film doped with thorium shows a high sensitivity towards trimethylamine (TMA) gas. It has been demonstrated that 1.18 wt.% thorium-doped tin oxide has a sensitivity of 2900 towards 800 vol. ppm TMA gas in air, while pure tin oxide has only a sensitivity of 5 at the operating temperature of 225 degreesC. Thorium stabilizes the tin oxide structure at lower deposition temperature, gives improved electrical characteristics and also enhances the sensitivity and selectivity towards TMA. The effect of surface coverage, morphology, oxidation state and amount of Th on the sensitivity and selectivity has been studied. The correlation of thorium doping in tin oxide at different deposition temperatures with morphology, improvement in the selectivity and sensitivity towards TMA is discussed. (C) 2003 Elsevier B.V. All rights reserved.