Transition-metal complexes, especially cobalt complexes, are used as precatalysts in homogeneous water oxidation catalysis. However, a practical method of identifying the effective active species in cobalt complex homogeneous catalysts under highly oxidizing conditions have not yet been established. In this work, (H3O)(6).[Co-4(H2O)(4)(HPMIDA)(2)(PMIDA)(2)].2H(2)O (1) was introduced, and its photocatalytic performance was evaluated. The compound exhibited effective water oxidation catalysis, with a turnover number of 661.5 at pH 9.0 using [Ru(bpy)(3)](ClO4)(2) as photosensitizers and sodium persulfate as sacrificial electron acceptor. Accordingly, 1 was used as target catalysts, and the active species was thoroughly investigated under catalytic conditions. The stability of 1 was further tested and confirmed by a series of experiments (cyclic voltammetry (CV), ultraviolet-visible (UV-vis) spectrometry, dynamic light scattering (DLS), aging experiments, and extraction techniques). Results showed that 1 maintained its structural integrity under the given photocatalytic conditions. Furthermore, cathodic adsorptive stripping voltammetry and inductively coupled plasma-mass spectrometry were used to quantify the amount of Co2+ ions released from I into borate buffer after oxidation. The pH dependence of the oxygen-evolution performance of 1 and other cobalt species were then compared. Data collectively revealed that 1 was stable during water oxidation; it did not release free Co2+ ions and was not hydrolyzed to cobalt oxide or hydroxide, thereby confirming that 1 was the effective active species in water-oxidation catalysis. (C) 2016 Elsevier B.V. All rights reserved.