Catalytic water oxidation is the crucial challenge in the conversion of sunlight energy into such green fuels as hydrogen from water: H2O + sunlight ® H2 + ½ O2. At present, the water oxidation catalysts (WOCs), based on abundant metals such as cobalt, are the most efficient and promising for further development. We have developed a family of Co-containing molecular WOCs containing different oxidatively robust polydentate polyoxometalate ligands with different hydrolytic stabilities and catalytic activities. Two representatives of this class of WOCs are the polyoxometalates (POMs) of formula [Co4(H2O)2(XW9O34)2]10-, X = PIV (1-P2) or VV (1-V2). While, they are isostructural with two of the Co(II) centres accessible to water, their activities and stabilities are very different. At present, the homogeneous molecular 1-V2 catalyst is by far the fastest WOC to date. New experimental data on stability of Co-based polyoxometalates will be reported. Kinetic models of catalytic water oxidation will be also discussed.