A binuclear manganese molecular complex [(OH2)(terpy)Mn(mu-O)(2)Mn(terpy)(OH2)](3+) (1) is the most prominent structural and functional model of the water-oxidizing Mn complex operating in plants and cyanobacteria. Supported on montmorillonite clay and using Ce(IV) as a chemical oxidant, 1 has been reported to be one of the best Mn-based molecular catalysts toward water oxidation. By X-ray absorption spectroscopy and kinetic analysis of the oxygen evolution reaction, we show that [(OH2)(terpy)Mn(mu-O)(2)Mn(terpy)(OH2)](3+) is transformed into layered type Mn-oxide particles which are the actual water oxidation catalyst.