Abstract Chloroplast thylakoid membrane protein phosphorylation inhibits Photosystem-II-mediated electron transfer at saturating light intensities. Measurements of room temperature chlorophyll fluorescence have been used to monitor electron transfer reactions between the primary (Q A) and secondary (Q B) quinone acceptors of Photosystem II. The initial transfer from Q − A to Q B is not affected by the phosphorylation, although a second slow reoxidation phase is even slower after phosphorylation. After equilibrium is achieved between Q − AQ B and Q AQ − B there appears to be a higher concentration of Q − A in the phosphorylated membranes. This is explained by a phosphorylation-induced destabilisation of the anionic semiquinone, Q − B, as seen from the DCMU-stimulated fluorescence increase.