The conduction process in nanostructured SnO2 gas sensors under an external voltage has been studied, in order to work out the main differences between fully depleted and not fully depleted grains. With a model based on the double Schottky barrier which occurs at the interface between two adjacent grains, it has been shown that the shape of the current-voltage curves is completely different in the two cases. The conduction carriers available in not fully depleted grains respond to the external electric field, contributing to the total electrostatic potential, this being a crucial difference from fully depleted grains. In this work, we determined the shape of the barrier in both cases and provided a calculation of the electrical current as a function of the external bias, including tunneling and thermionic contributions. Theoretical simulations of current-voltage characteristics have been compared with experimental data. (C) 2013 Elsevier B.V. All rights reserved.