In this work the influence of oxidation of a SnO2 thin film on the development of a depletion layer was studied by using computer simulations. The surface potential and in-depth potential profiles in the depletion region were calculated by solving the Poisson–Boltzmann equation for finite SnO2 flat grains (partially or totally depleted) using slab geometry. The effect of the donor (oxygen vacancies in the bulk) mobility on electrical potential inside grains with different thicknesses was discussed. Furthermore, in-depth profiles of mobile donor Nd(r) and electron n(r) concentrations in the depletion layer were computed for different densities of negative charge trapped in the surface states, as a result of oxygen adsorption. It was assumed that the bulk oxygen vacancies (donors) are singly ionized and mobile.