A theoretical model has been developed to interpret the size dependent behavior of nanostructured metal-oxide semiconductors. It is based on the determination of the surface-state density, which pins the Fermi level of the semiconductor, thus removing the linear relationship between the work function and the Schottky barrier. To provide the model with numerical input, measurements of the Schottky barrier height were performed at different temperatures on nanocrystalline SnO2 and TiO2 films. The obtained solution predicts the unpinning of the Fermi level when the material can be considered as nanostructured, and the evidence confirms it.