SnO2 films are commonly used as gas sensors. An important environmental application is the use and development of SnO2 thin films in order to detect low concentrations of toxic gases (i.e., CO, NO2, O-3,...) in urban environments. The gas response of the SnO, sensors strongly depends on the preparation process and especially on deposition parameters. SnO2 thin films deposited by reactive sputtering are, in general, highly sensitive and selective above all towards CO and NO2, even at very low concentrations. These sensors also present short response times to,eases, as well as good reproducibility and repeatability. The gas detection capability of these sensors is based on the variations of the sensor resistance caused by the adsorption of gas on the sensor surface. This gas adsorption involves, above air, the first layers of the SnO2 films. The aim of this work was to correlate the chemical composition focusing on the first layers and structural properties of SnO2 films with their capability to detect gas. SnO2 thin films were prepared with varying sputtering deposition parameters (i.e., the percentage of O-2 in the Ar sputtering plasma and the substrate temperature). The chemical composition of the first layers of SnO2 films was examined by X-ray Photoelectron Spectroscopy (XPS). The morphology and the long and short range orders of SnO2 thin films were analysed by Glancing Angle X-Ray Diffraction (GAXRD) and by Extended X-ray Absorption Fine Structure (EXAFS) at the Sn-K edge. The results showed that SnO2 films deposited at room temperature (RT) and with 6-8% of O-2 in the Ar sputtering plasma were the better gas sensors displaying high sensitivity to the gases, good reproducibility and reliability. These samples had an average SnO2 grain size of 30-50 Angstrom and a high percentage of adsorbed oxygen in the first two layers of SnO2 film.