It has been reported that addition of pol yoxo metal ates (POM) or fluoride anions into the TiO2 dispersions can significantly enhance the photocatalytic degradation (PCD) of weakly adsorptive organic pollutants in water such as chlorophenol. In this work, however, contradictory effects of POM and fluoride were observed on the PCD of highly adsorptive substrate X3B, an anionic organic dye, under similar conditions. The total rate of X3B PCD, determined by total loss of X3B both in solution and on the catalyst surface, was increased in the presence of fluoride, but the rate was decreased in the presence of POM. In both cases, the dark adsorption of X3B on TiO2 was greatly decreased, ascribed to competitive adsorption of POM or fluoride that reduces the positive charges on the catalyst surface. The spectral analysis and the kinetic study using tert-butyl alcohol as hydroxyl radical scavenger revealed that the PCD of X3B on naked TiO2 was predominately initiated by direct hole transfer, whereas addition of POM or fluoride into the TiO2 dispersions enhanced the degradation of X3B via hydroxyl radical pathway. It is proposed that the surface occupation of POM on TiO2 accelerates the production of surface-bound hydroxyl radicals, due to enhanced charge separation, whereas the fluoride replacement of surface hydroxyl groups of TiO2 increases the production of free hydroxyl radicals in solution, due to enhanced hole availability for water oxidation. Assume that the relative reactivity among various active follows the order of free hydroxyl radicals > subsurface holes > surface-bound hydroxyl radicals, the proposal could account for the observed effects of POM and fluoride on the PCD of both weakly and highly adsorptive organic substrates over TiO2, such as chlorophenol and X3B.