Photocatalytic water splitting into H-2 and O-2 Over various tantalates was reviewed and factors affecting photocatalytic activities of tantalates were discussed from a viewpoint of the crystal structure and the energy structure. Many tantalates, K3Ta3Si2O13, alkali and alkaline earth tantalates, were highly active photocatalysts for water splitting. The high activities were mainly due to the high conduction band level consisting of Ta5d orbitals. NiO-loaded NaTaO3 with the distorted perovskite structure showed the highest activity among tantalates. Moreover, the activity of the NiO/NaTaO3 photocatalyst was remarkably improved by doping of lanthanoids. The apparent quantum yield of NiO/NaTaO3 doped with lanthanum was ca. 50% at 270 nm.