BiVO4 photocatalysts for O-2 evolution, which work under visible light irradiation, were prepared by an aqueous process. The BiVO4 photocatalysts were obtained by the reaction of layered potassium vanadate powder (KV3O8 and K3V5O14) With Bi(NO3)(3) for 3 days in aqueous media at room temperature. Highly crystalline monoclinic and tetragonal BiVO4 were selectively synthesized by changing the ratio of vanadium to bismuth in the starting materials. X-ray diffraction and scanning electron microscopy measurements showed that the monoclinic BiVO4 was formed via a tetragonal BiVO4 intermediate. Tetragonal BiVO4 with a 2.9 eV band gap mainly possessed an ultraviolet absorption band while monoclinic BiVO4 with a 2.4 eV band gap had a characteristic visible light absorption band in addition to the UV band. The UV bands observed in the tetragonal and monoclinic BiVO4 were assigned to the band transition from O-2p to V-3d whereas the visible light absorption was due to the transition from a valence band formed by Bi-6s or a hybrid orbital of Bi-6s and O-2p to a conduction band of V-3d. The photocatalytic activity for O-2 evolution from an aqueous silver nitrate solution under UV irradiation (300 < lambda < 380 nm) on the tetragonal BiVO4 was comparable to that on the monoclinic BiVO4. The monoclinic BiVO4 also showed the high photocatalytic activity for the O-2 evolution under visible light irradiation (lambda > 420 nm). When the monoclinic BiVO4 was calcined at 700-800 K the activity was increased. The activity of this monoclinic BiVO4 was much higher than that of BiVO4 prepared by a conventional solid-state reaction. The quantum yield at 450 nm for the O-2 evolution on the monoclinic BiVO4 was 9%.