Photochemical quartz crystal microbalance (PQCM) studies of nanocrystalline titanium dioxide films show evidence of charge-compensating cation intercalation during photolytically induced accumulation layer formation. The manifestation and magnitude of mass uptake (intercalation) are dependent, respectively, on the illumination and intensity of illumination provided. Isotope experiments performed in H2O and D2O unambiguously identify the cations as protons and deuterons. The experiments extend earlier experiments with dark TiO2 (and other metal oxide semiconductors) that have implicated cation intercalation as the primary process involved in charge compensation under accumulation conditions.