Herein, water-oxidation reaction by cobalt(II) phthalocyanine, N,N'-bis (salicylidene) ethylenediamino cobalt(II), nickel(II) Schiff base (N,N'-bis (salicylidene)ethylenediamino nickel(II), nickel(II)) phthalocyanine-tetrasulfonate tetrasodium, manganese(II) phthalocyanine, 5,10,15,20-Tetraphenyl-21H,23H-porphine manganese(III) chloride, manganese(III) 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine chloride tetrakis(methochloride) was investigated using electrochemistry, UV-vis spectroscopy and spectroelectrochemistry. According to our results, a new decomposition pathway for these metal complexes under water-oxidation conditions was proposed. The produced metal oxide obtained by decomposition of metal complex under water -oxidation conditions not only catalyzes water-oxidation reaction but this metal oxide also accelerates decomposition of the corresponding complex to form higher amounts of the metal oxide. We hypothesize that such a mechanism could be investigated for many metal complexes under different oxidation or reduction reactions.