Thin film nanocrystalline tin dioxide sensors have been prepared by reactive sputtering in order to test them at different oxygen partial pressures. Variations on the him conductivity covers almost two orders of magnitude and, in the range studied, obeys a power law relationship. The results are discussed in the framework of the electron theory of adsorption where the mean parameters are grain size, surface trap energy level and number of chemisorption sites. Variations of the last two parameters with temperature result in a very good agreement between predicted and observed values of sensitivity.