Oxygen ligands from water are bound to the manganese cluster giving rise to the 'multiline' EPR signal from the S 2 state of photosystem II, as shown by the observation of hyperfine broadening of the EPR signal in the presence of 1 7 O-enriched water. The binding must occur in the S 0 , S 1 or S 2 state. The estimated upper limit for the 1 7 O hyperfine coupling constant, 0.5 mT, excludes Superoxide and hydroxyl radicals in the S 2 state, but not peroxide.