The adsorption of different gases onto tin oxide films surfaces promotes electrical conductance variations. Films chemiresistive properties depend on the intergranular barrier heights and on the concentration of non-stoichiometric defects that, in the case of tin oxide, are mainly oxygen vacancies. We found that film exposures to an H-2-containing atmosphere over 400 A degrees C led to unusual results that cannot be explained resorting to regular interpretations. By means of Raman spectroscopy and XRD characterization, we show that, at high enough temperatures, when films are exposed to hydrogen, the tin dioxide converts into tin monoxide and metallic tin. Then, when films are exposed to air atmosphere, the oxygen diffuses into the grains converting the metallic tin back into tin dioxide. These findings are consistent with electrical measurements.