The observed adsorption of methylene blue, MB + , on P25 titania over a range of pH values (pH 2–11) is reported and the results fitted to a charge distribution, multisite complexation, i.e. CD-MUSIC, model, modified for both monomer and aggregated dye adsorption and (for aggregates) co-adsorption of hydroxyl ions. The rate of bleaching of MB + , photocatalysed by titania using UV light, increases over the pH range 4–11 in much the same way the amount of adsorbed MB + , [MB + ] ads , increases, indicating that the rate-determining step involves direct hole oxidation of adsorbed MB + . In contrast, at pH 2, the rate of UVA dye photobleaching appears unduly high but is due to an additional reductive, as well as the usual oxidative, bleaching processes. The initial rate of bleaching of MB + via a visible light-driven, dye-sensitised process, measured at different pH's correlates with the associated CD-MUSIC-calculated values of [MB + ] ads , indicating that only dye molecules that are in direct contact with the surface can undergo photodegradation via a dye-sensitised route.