Gas sensors based in semiconductor oxides show a change in the resistance when they are exposed to certain gaseous atmospheres. In this paper, the influence on the sensor electrical resistance with the film thickness in vacuum and air are studied. The conduction process is analyzed considering the existence of Schottky potential barriers at the grain boundaries. These barriers are affected by the intergranular gas diffusion. This process increases the barrier height and width, altering the electrical resistance. Also, the influence of the diffusion process on the stabilization time is analized.