Cl- and Ca2+ are obligatory cofactors in photosystem II (PS-II), the oxygen-evolving enzyme of plants. The electron transfer events in Cl(-)-depleted PS-II were investigated by using continuous wave (cw) and pulsed EPR, and the results were compared with those obtained in untreated and Ca(2+)-depleted PS-II. In Cl(-)-depleted PS-II, the S1 to S2 transition is not accompanied by the appearance of the S2 manganese signal in both cw and field-swept echo experiments. In the S3 state, the cw experiment reveals the presence of a radical signal, and the field-swept echo experiment reveals the presence of a manganese signal similar to that observed in the S2 state in Cl(-)-reconstituted PS-II. A relaxation enhancement study of the oxidized tyrosine D indicates that a relaxation enhancement occurs only in the S2 to S3 transition and not in the S1 to S2 transition. The results are interpreted by the following preferred model. In Cl(-)-depleted PS-II, the S1 to S2 transition corresponds to the oxidation of a component other than the manganese cluster, possibly an amino acid, and the S2 to S3 transition corresponds to the oxidation of the manganese complex. This oxidation sequence is the opposite of that observed in Ca(2+)-depleted PS-II and may be due to a modulation by chloride of the relative redox potentials of the manganese cluster and a nearby oxidizable amino acid. An alternative model involving manganese oxidation that is invisible in the S1 to S2 transition but that becomes visible on S3 formation cannot be ruled out, although it is considered less likely.