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Gas sensing is the main application of tin dioxide besides transparent coatings for solar cells and displays and besides catalysis. SnO2-based sensors are applied in many (industrial) areas including automotive, chemical, environmental control, food, medicine, military, and safety. Excellent sensor performance is often reported for quite synthetic environments, while under real working conditions the metal oxide-based sensors still suffer from high cross-sensitivities. The high sensitivity for a particular gas and the simultaneously low selectivity are related to the working principle of metal-based sensors. Therefore, a new methodology of investigation and modelling has been defined, based on the simultaneous application of several complementary methods under conditions as close as possible to the real working conditions to obtain a synergetic effect for the indication of the sensor effect of the different surface reactions. When DRIFT (diffuse reflectance infrared Fourier transformed) spectroscopy was used to investigate the mechanism of CO on undoped and PD-doped tin dioxide sensors, spectroscopic and electrical measurements were simultaneously performed, while input information on water and oxygen absorption was still based on experiments performed under different conditions. Here, the mechanism of propane interaction with SnO2 sensors in air was investigated by simultaneous DRIFT and DC (direct current) resistance studies, supplemented by simultaneous work function measurements and conversion analysis for the understanding of the mechanism of propane sensing. From the results, a preliminary mechanism was proposed of propane reaction with tin dioxide sensors under real working conditions, including adsorbed propyl radical and propanoate species. Then, dedicated experiments were performed for CO sensing under different oxygen concentrations from very low (30 ppm) up to 20.5 % (10,000 ppm). Finally, the effects of different sensor technologies (different fabrication, loading, post-treatment) on the CO sensing mechanism of thick film sensors based on hydrothermally treated powders were established. The structure of surface hydroxyl groups varied, mainly induced by SnO2 powder preparation procedures. All aspects concerning surface oxygen (adsorbed or in the lattice) were nontrivial. As long as there were sufficient oxygen ions at the surface, the hydroxyl groups did not change upon CO exposure, while under oxygen lean conditions the number of hydroxyl ions decreased, and the conversion of CO to CO2 was blocked. Thus in order to increase the sensitivity for CO, more reactive oxygen species must be delivered by proper tuning of the sensor preparation procedure. The effects of post treatment seem to be stronger than the effects of Pd sensitisation, and therefore the role of Pd could not be clarified.