The optical and electrochemical properties of nanocrystalline TiO2 electrodes in acidic aqueous solution are reported. Cyclic voltammetry, UV/vis/NIR attenuance, EPR, photocurrent spectroscopy, and electrochemical impedance measurements are presented. At negative applied potentials the TiO2 films turn black, and an EPR spectrum appears which we attribute to Ti(III) species. The results are described in terms of a model in which the reduction of TiO2 leads to band edge unpinning.