Photocatalytic activity of lanthanide tantalates, LnTaO(4) (Ln = La, Ce, Pr, Nd, and Sm), for water splitting was studied in connection to the effect of Ln 4f levels on the electronic structure. Valence band XPS, UV-vis. and first-principle electronic calculations suggested that the position of Ln 4f levels becomes lowered monotonically across the series of Ln. The empty La 4f level is supposed to be higher than the conduction band edge, whereas the filled 4f levels of the Nd and Sm compounds lie in the valence band, For Ln = Ce and Pr, occupied 4f levels lying within the forbidden gap lead to the lower band gaps (E-g) compared to others. The highest photocatalytic activity was attained by LaTaO4, which solely can decompose water into stoichiometric H-2/O-2 mixtures. This is largely due to the overlap between Ta 5d and O 2p orbitals, which produces a wide density-of-states distribution in the conduction band. On the other hand, the tantalates containing from one (Ce3+) to five (Sm3+) 4f electrons were much less active, because unoccupied 4f levels lying on or below the conduction band edge would play as a trapping center for photoexcited electrons. The interactions between the empty and filled Ln 4f levels and carriers generated under UV irradiation are possible reasons for the Ln-dependent photocatalytic activity.