In this study, the influence of the co-existence of TiO(2) nanoparticles on the speciation of arsenite [As(III)] was studied by observing its adsorption and valence changing. Moreover, the influence of TiO(2) nanoparticles on the bioavailability of As(III) was examined by bioaccumulation test using carp (Cyprinus carpio). The results showed that TiO(2) nanoparticles have a significant adsorption capacity for As (III). Equilibrium was established within 30 min, with about 30% of the initial As (III) being adsorbed onto TiO(2) nanoparticles. Most of aqueous As (III) was oxidized to As(V) in the presence of TiO(2) nanoparticles under sunlight. The carp accumulated considerably more As in the presence of TiO(2) nanoparticles than in the absence of TiO(2) nanoparticles, and after 25-day exposure, As concentration in carp increased by 44%. Accumulation of As in viscera, gills and muscle of the carp was significantly enhanced by the presence of TiO(2) nanoparticles.