A series of experiments was performed to investigate the observed changes in conductivity when sputtered tin oxide films maintained at 525 K are exposed to various ambient gases. X-ray photoemission spectroscopy (XPS) measurements on quenched samples monitored changes in both the chemical state and stoichiometry which occurs during exposure to O2, H2, NO, and H2S. The XPS results were correlated with in situ measurements of sample resistance. This comparison demonstrated no direct connection between the concentration of an adsorbed impurity, i. e., N or S, and conductivity. However, the bulk concentration of oxygen atoms is strongly correlated with the sample conductivity. We conclude that the predominant response mechanisms involve the oxidation/reduction of the sample with a concommitant decrease/increase in the bulk concentration of oxygen vacancies which mediates the conductivity.