Presently, many test results tend to prove that the effects of a gas on the electrical properties of tin dioxide are rather complex. Some aspects, such as the presence of a peak on the conductance curves as a function of temperature, may be explained by the combination of several regulating effects such as potential barriers at grain boundaries or the material's intrinsic resistance. This enabled us to state precisely the part played by some morphological aspects, such as grain size, on the electrical performance of the material. Sintered materials obtained through the compression of a commercial powder, as well as materials produced in the form of thin films through the CVD technique (Chemical Vapour Deposition), are considered in this study. The results obtained demonstrate that the preparation and annealing conditions play a major part in the nature and amplitude of the effects observed with methane and alcohol. It should be noted, in particular, that neither the amplitude nor the position of the conductivity peak as a function of the temperature can be varied. Through the differences observed between methane and alcohol, it is possible to conclude that surface chemical reactions are more particularly located at grain boundaries.