In natural photosynthesis, the oxygen evolving center is a tetranuclear manganese cluster stabilized by amino acids, water molecules and counter ions. However, manganese complexes are rarely exhibiting catalytic activity in water oxidation conditions. This is also true for the family of water oxidation catalysts (WOCs) obtained from POM chemistry. We have studied the activity of the tetranuclear manganese POM [Mn4(H2O)2(PW9O34)2]10—(Mn4), the manganese analog of the well-studied [Co4(H2O)2(PW9O34)2]10— (Co4), one of the fastest and most interesting WOC candidates discovered up to date. Our electrocatalytic experiments indicate that Mn4 is indeed an active water oxidation catalysts, although unstable. It rapidly decomposes in water oxidation conditions. Bulk water electrocatalysis shows initial activities comparable to those of the cobalt counterpart, but in this case current density decreases very rapidly to become negligible just after 30 min, with the appearance of an inactive manganese oxide layer on the electrode.