The mechanism of the decomposition of a water molecule which penetrates into the coordination sphere of the active centre of the complex is discussed. The interaction results in the reaction: H 2 O → H + + e + OH . . The electron reduces the Mn 3 + ion in the complex to Mn 2 + , the proton transfers to a second water molecule bound to the first by a hydrogen bond and the OH . radical interacts with the xanathophylls, subsequently leading to liberation of an oxygen molecule.