36C1-was used to study the slow exchange of chloride at a binding site associated with Photosystem II (PS II). When PS II membranes were labeled with different concentrations of 36C1-, saturation of binding at about 1 chloride/PS II was observed. The rate of binding showed a clear dependence on the concentration of chloride approaching a limiting value of about 3-10 -4 s ~ at high concentrations, similar to the rate of release of chloride from labeled membranes. These rates were close to that found earlier for the release of chloride from PS II membranes isolated from spinach grown on 36C1-, which suggests that we are observing the same site for chloride binding. The similarity between the limiting rate of binding and the rate of release of chloride suggests that the exchange of chloride with the surrounding medium is controlled by an intramolecular process. The binding of chloride showed a pH-dependence with an apparent pK a of 7.5 and was very sensitive to the presence of the extrinsic polypeptides at the PS II donor side. The binding of chloride was competitively inhibited by a few other anions, notably Br-and NO 3. The slowly exchanging C1-did not show any significant correlation with oxygen evolution rate or yield of EPR signals from the S 2 state. Our studies indicate that removal of the slowly exchanging chloride lowers the stability of PS II as indicated by the loss of oxygen evolution activity and S 2 state EPR signals.