In acidic aqueous solutions the protonated TiO2 colloid undergoes a complex process of electroreduction. It starts by reduction of protons on the surface of the particles; in cyclic voltammetry with a fast scan rate this reaction is reversible. In the absence of an excess of a supporting electrolyte the particles, deprived of the stabilizing positive charge in the electroreduction process, aggregate and adsorb at the electrode and hinder the electrode reaction. Added electrolyte removes this hindrance and produces an increase of the electrolytic current. The hydrogen atoms generated at the TiO2 surface are able to reduce Ti(TV) to-Ti(III). At more negative potentials a direct reduction of surface hydroxo complexes of Ti(IV) takes place. The mechanism and the potential of electroreduction of TiO2 colloids are strongly affected by ligands which form complexes with titanium atoms on the particle surface.