The Clbinding properties in the successive oxidation states of the O 2 evolving complex of photosystem II were investigated by measurements of UV absorbance changes, induced by a series of saturating flashes, that monitor manganese oxidation state transitions. In dark-adapted, intact photosystem II, Clcan be replaced by NO 3in minutes, in an exchange reaction that depends on the NO 3concentration and that is not rate-limited by dissociation of Clfrom its binding site. Preillumination of dark-adapted photosystem II by one or two flashes accelerated the NO 3substitution reaction by an order of magnitude. A quantitative analysis of the Clconcentration dependence of UV absorbance changes, measured in photosystem II preparations depleted of extrinsic 17 and 23 kDa polypeptides, shows that the Clbinding properties of photosystem II change with the oxidation state of the oxygen evolving complex. Although the affinity for the individual S-states could not be determined with precision, it is shown that the affinity is an order of magnitude lower in the S 2 state than in the S 1 state. Comparison of the results obtained using intact photosystem II and preparations depleted of the 17 and 23 kDa extrinsic polypeptides suggests that these proteins constitute a diffusion barrier, which prevents fast equilibration of the Clbinding site with the medium, but does not change the Claffinity of the binding site.