We used the novel experimental technique of photoemission spectromicroscopy to investigate the active area of SnO(x) thin films for gas sensors, deposited by dc sputtering. First, we analyzed the degree of oxidation and the homogeneity of as-deposited films in a high lateral resolution (30 mum) spectromicroscopy mode with an energy resolution of 0.4 eV. This led to a surprising discovery of a large amount of tin monoxide on the film surface. Then we studied the interaction of H-2 and H2O with the surface of polycrystalline SnO2 films used as sensitive layers in actual gas sensor devices. These results have been related to the resistivity changes of the corresponding devices: we found that such changes are primarily due to a reduction process in the film rather than to mere chemisorption.