Addn. of 4-tert-butylpyridine (4TBP) to redox electrolytes used in dye-sensitized TiO2 solar cells has a large effect on their performance. In an electrolyte contg. 0.7M LiI and 0.05M I2 in 3-methoxypropionitrile, addn. of 0.5M 4TBP gave an increase of the open-circuit potential of 260 mV. Using charge extn. and electron lifetime measurements, this increases could be attributed to a shift of the TiO2 band edge toward neg. potentials (responsible for 60% of the voltage increase) and to an increase of the electron lifetime (40%). At a lower 4TBP concn. the shift of the band edge was similar, but the effect on the electron lifetime was less pronounced. The working mechanism of 4TBP can be summarized as follows: (1) 4TBP affects the surface charge of TiO2 by decreasing the amt. of adsorbed protons and/or Li+ ions. (2) It decreases the recombination of electrons in TiO2 with triiodide in the electrolyte by preventing triiodide access to the TiO2 surface and/or by complexation with iodine in the electrolyte.