The potential-dependent optical-absorption spectroscopy of nanostructured TiO2 (anatase) is most fully accounted for by two processes, band-filling and intercalation. Under weak-accumulation conditions, occupation of surface and conduction-band states by electrons dominates with the accumulated charge being compensated by adsorbed protons or cations. Under strong-accumulation conditions, however, intercalation of protons or cations may take place with a fraction of the accumulated electrons being localized at Ti-IV sites.