Earth-abundant cobalt oxides as active water oxidation catalysts have attracted extensive attention, but the controlled synthesis of small cobalt oxide clusters still remains challenging. Herein we introduce a novel approach to deposit small cobalt oxide clusters on alumina which involves mild oxidation of molecularly dispersed Co-4(CO)(12) precursor. Scanning transmission electron microscope (STEM) shows that the size of prepared CoOx clusters is smaller than 2 nm. Turnover frequency (TOF) value of water oxidation on the synthesized cobalt oxide clusters can reach as high as 0.067 s(-1) in photochemical Ru(bpy)(3)(2+)-Na2S2O8 system, which is at least an order of magnitude higher than bulk cobalt oxide nanoparticles. The remarkably enhanced activity of cobalt oxide cluster is attributed to its more surface-exposed cobalt atoms and higher intrinsic activity. (C) 2015 Published by ElseVier B.V.