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The active site for water oxidation in photosystem II (PSII) consists of a Mn 4 Ca cluster close to a redox-active tyrosine residue (TyrZ). The enzyme cycles through five sequential oxidation states (S 0 to S 4 ) in the water oxidation process. Earlier electron paramagnetic resonance (EPR) work showed that metalloradical states, probably arising from the Mn 4 cluster interacting with TyrZ, can be trapped by illumination of the S 0 , S 1 and S 2 states at cryogenic temperatures. The EPR signals reported were attributed to S 0 TyrZ, S 1 TyrZ and S 2 TyrZ, respectively. The equivalent states were examined here by EPR in PSII isolated from Thermosynechococcus elongatus with either Sr or Ca associated with the Mn 4 cluster. In order to avoid spectral contributions from the second tyrosyl radical, TyrD, PSII was used in which Tyr160 of D2 was replaced by phenylalanine. We report that the metalloradical signals attributed to TyrZ interacting with the Mn cluster in S 0 , S 1 , S 2 and also probably the S 3 states are all affected by the presence of Sr. Ca/Sr exchange also affects the non-haem iron which is situated approximately 44 Å units away from the Ca site. This could relate to the earlier reported modulation of the potential of Q A by the occupancy of the Ca site. It is also shown that in the S 3 state both visible and near-infrared light are able to induce a similar Mn photochemistry.