Various Ln(a)/Ln(b) co-doped bismuth molybdate (Bi2MoO6) photocatalysts were synthesized by a hydrothermal process. The result of photocatalytic experiment demonstrated that the relevant Ln(1)(3+) 4f(7+x)/Ln(2)(3+) 4f(7-x) (Ln(1)/Ln(2) = Tb/Eu, Dy/Sm, Er/Nd; x = 1, 2, 4) co-doped Bi2MoO6 samples shows higher photocatalytic activities. Moreover, all the photo catalytic activities of Ln(1)/Ln(2) co-doped Bi2MoO6 were higher than that of the corresponding single-doped Bi2MoO6. The crystalline structures, morphology, constituent contents, chemical state and optical properties of the samples were analyzed in detail. Meanwhile, a new conjecture about the complementary distribution of 4f orbital electrons in the Ln(1)/Ln(2) redox couple was proposed through co-doping with Ln(1) and Ln(2) ions to improve the photocatalytic activity of Bi2MoO6. For the Bi2MoO6 photcatalyst, Dy/Sm co-doping displayed the strongest photocurrent response as well as the best photocatalytic activity, while Tb/Eu co-doping displayed the apparently extended visible-light absorption region. These findings provides a novel strategy to fabricate and engineer high-efficiency photocatalysts by selecting appropriate Ln(1)/Ln(2) redox couple. (C) 2017 Elsevier B.V. All rights reserved.