The oxygen-evolving complex, (OEC) also known as the water-splitting complex, is a water-oxidizing enzyme involved in the photooxidation of water during the light reactions of photosynthesis. It is surrounded by 4 core proteins of photosystem II at the membrane-lumen interface. Based on a widely accepted theory from 1970 by Kok, the complex can exist in 5 states: S₀ to S₄. Photons trapped by photosystem II move the system from state S₀ to S₄. S₄ is unstable and reacts with water producing free oxygen. Currently, the mechanism of the complex is not completely understood. Much of the known data have been collected from flash experiments, EPR, and X-ray spectroscopy.