We report the synthesis and photoelectrochemical activity of three lanthanide-titanium oxo clusters (LTOCs), formulated as [Ln(8)Ti(10)(mu(3)-O)(14)(tbba)(34)(Ac)(2)(H2O)4(THF)(2)]center dot 2Htbba [Ln = Eu (1), Sm (2), and Gd (3); Htbba = 4-tert-butylbenzoic acid; Ac- = acetate]. These stable compounds are efficient catalysts of photoelectrochemical water oxidation with high turnover numbers (7581.0 for 1, 5172.4 for 2, and 5413.0 for 3) and high turnover frequencies (2527.0 for 1, 1724.1 for 2, and 1804.0 for 3). The differences in the photo electrochemical activity among these three compounds may be related to the differences in their band gaps. This work shows that the heterometallic LTOCs provide a tunable platform for the design of highly effective water oxidation catalysts.