The diffusional permeability of I-3(-) ion in acetonitrile in free standing TiO2 membrane with a porosity of 55% was examined. The apparent diffusion coefficient, D-app at 25 degrees C of the ion was found to be 3.4 x 10(-6) cm(2 -1), an order of magnitude smaller than the free diffusion at the same temperature. The temperature dependency of D-app was measured in the range 0-30 degrees C and analysed in terms of the Walden product. The diffusional activation energy was found to be 13.5 kJ/mol. The parameters of interest for the efficiency of mesoscopic wet solar cells are discussed. A back of an envelope calculation shows that although the obstructed diffusion coefficient of the I-3(-) ion was an order of magnitude smaller than the free diffusion the diffusional flux is still sufficient to meet a current density of 50 mA cm(-2). At incident photon flux of 1 kW m(-2) and at a photopotential of 0.6 V this would correspond to a solar energy efficiency of approximately 30%.