A model for the electronic interaction between additives and semiconducting oxide gas sensors is presented. The model is based on the depletion layer that is created due to the metal–semiconductor contact forced by the presence of a metallic additive. This depletion layer corresponds to an active grain size that is smaller than the geometrical one. Thus, the work function of the metal is connected to the change of sensor characteristics. Moreover, the dependence of the sensitivity on the amount of the deposited additive is explained on the same basis. Experimental results are presented in the case of tin oxide thin film gas sensors with Pd, Pt, and Ni in the presence of zero grade air and carbon monoxide. The experimental results are in excellent qualitative agreement with the proposed model. ©1996 American Institute of Physics.