The reaction path of the evolution of triplet oxygen molecule from [Mn4(III)-OOH] at the S-4 state of the catalytic cycle of the oxygen evolving complex has been explored theoretically. The activation energy is only 2.7 kcal mol(-1). The produced S-0 state has the oxidation state of Mn-4(III, IV, IV, II) with reduction from Mn-4(III) to Mn-4(II). Our proposed path is consistent with the experimental observation.