Our recent EPR and EXAFS experiments investigating the structure of the oxygen-evolving complex of PS II are discussed. PS II treatments which affect the cofactors calcium and chloride have been used to poise samples in modified forms of the S-states, S1, S2 and S3. X-ray absorption studies indicate a similar overall structure for the manganese complex between treated and native samples although the influence of the treatments and cofactors is observed. Manganese oxidation (or oxidation of a ligand to the manganese cluster) is indicated to occur on each of the transitions S1 โS2 and S2 โS3 in these modified samples. The cluster appears to contain at least two inequivalent Mn-Mn pairs. In the native samples the Mn-Mn distance is 2.7 ร
, but in samples where the calcium site is affected, one of the pairs has a 3.0 ร
Mn-Mn distance. The intensity of the 3.3/3.6 ร
interaction is reduced on sodium chloride treatment (calcium depletion) perhaps indicating calcium binding close to the manganese cluster. From EPR data we also propose that treatments which affect calcium and chloride binding cause a modification of the native S2 state, slow the reduction of Yz (โข) and allow an S3 EPR signal to be observed following illumination. The origin of the S3 EPR signal, a modified S3 or S2 X(โข) where X(โข) is an organic radical of unknown charge, is discussed in relation to the results from the EXAFS studies.