Thin films of sputtered In,03 were exposed to a continuous flow of N-2, pure synthetic air and synthetic air contaminated with NO2, NH3 and H-2 while an electric field step function of 76 V/cm was applied. After the field parallel to the surface was switched on, large changes in the ratio between the film resistance close to the negative contact and the overall film resistance were observed. During the electric pulse different time-dependent behaviors were observed depending on the film ambient. This effect is attributed to migration phenomena on the surface and allows to build a new class of chemical sensors. It might, with this technology, be possible to distinguish different gases by means of the same sensitive film.