A model for the analysis of interference reflection data of nanoporous TiO 2 dye-sensitized solar cells (DSSC) is presented. It is shown that the interference reflection technique [Turrión, M.;Macht, B.;Tributsch, H.; Salvador, P. J. Phys. Chem. B 2001, 105, 9732] allows one to monitor very precisely the evolution of the depletion layer of the fluor-doped SnO 2 (FTO) conducting substrate at the DSSC back contact with the position of the Fermi level. The model shows that this technique features a much larger sensitivity than the capacitancevoltage Mott-Schottky method for determining both the flatband potential (U 0 ) and the Helmholtz layer capacitance (C H ) at the FTO/electrolyte interface, under working conditions of the DSSC. Under illumination the band-bending in the FTO substrate is controlled by two factors: (a) the position of Fermi level (FTO bulk potential), which is determined by the accumulation of photogenerated electrons in the conduction band of the nanostructured TiO 2 film, and (b) the values of U 0 and C H , which determine the potential distribution at the FTO/electrolyte interface. It is suggested that both parameters U 0 and C H determine a constraint on the maximum photovoltage attainable by the DSSC, as the conduction band edge of the FTO at the FTO/TiO 2 interface cannot be higher than that of the TiO 2 , otherwise the transfer of dye photoinjected electrons from the TiO 2 to the FTO would be hindered. According to our analysis, it can be concluded that the theoretical maximum photovoltage cannot be higher than 0.8 V, a value never surpassed experimentally with untreated DSSC.