The cathodic electrochemical behavior of nanosized sols of TiO2 on mercury electrodes in aqueous perchloric acid consists of two processes. The first, at more positive potential, is ascribed to the reduction of soluble Ti(IV) species. The nature of the second process, which does appear to involve the sole directly, has not been determined. However, it may involve the reduction of hydrogen ion adsorbed on the particles, as suggested by Heyrovsky, Jirkovsky, and Struplova-Bartackova.