The photocatalytic hydrogen (H-2) generation by the high nuclearity Co substituted polyoxometalates (PoMs), K10Na12[{Co-3(B-beta-SiW9O33(OH))(B-beta-SiW8O29 (OH)(2))}(2)]center dot 49H(2)O (abbreviated as CoPOM) was reported. Owing to the multielectron redox capabilities of the high nuclearity CoPOM, the POM showed excellent photocatalytic activities toward H-2 evolution in both molecule scale (homogeneous) and composite (heterogeneous) systems. The photocatalytic activities of CoPOM were much better than H3PW12O40. UV-Vis-NIR absorption spectral, Raman spectral, cyclic voltammetric behavior and intermittent photoelectrochemical current response were used to characterize the structure of the TiO2/CoPOM composite and interaction between TiO2 and CoPOM. Analogous "Z-scheme" and "dye" sensitized mechanisms were proposed for the homogeneous and heterogeneous systems toward photocatalytic H-2 evolution under solar light irradiation, respectively. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.