The properties of thin blocking layers of titanium dioxide used to improve the performance of dye-sensitized nanocrystalline solar cells have been studied. TiO2 blocking layers prepared on fluorine-doped tin oxide-coated glass by spray pyrolysis have been characterized by electrochemical impedance spectroscopy, spectroscopic ellipsometry, and voltammetry. The impedance data reveal the presence of a distribution of surface states at the titanium dioxide-electrolyte interface that is similar to the one seen in the case of nanocrystalline TiO2 films. The influence of the blocking layers on the back transfer of electrons to tri-iodide ions in electrolyte-based dye-sensitized nanocrystalline cells has been studied by open circuit photovoltage decay. The results show that the ability of the blocking layer to prevent the back reaction of electrons with tri-iodide ions in the electrolyte is excellent under short circuit conditions, but is limited under open circuit conditions by electron accumulation at the surface of the titanium dioxide blocking layer.