A model for the charge transfer involving a tunnelling process and considering the semiconductor character of SnO2 accounts well for the shape of the cd-η curves obtained for as-sprayed as well as for Ir-grafted SnO2. The role of Ir aggregates (1016 atoms cm−2 from Rutherford backscattering; 10–20% surface coverage from TEM observations) is two-fold: (i) to create a high density of surface states which freezes the band bending inside the film; and (ii) to increase the degeneracy of the SnO2 material at the very surface (bulk carrier concentration: 2.2 ×1020 cm−3), “short-circuiting” the tunnelling process. Then after Ir grafting, the relevant phenomenon is rather to replace the slow kinetics at bare SnO2 by a faster transfer rate at the metal-covered surface.