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Illumination of isolated Photosystem II reaction centres in the presence of the electron acceptors, silicomolybdate (SiMo) or 3,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), leads to selective photooxidation and irreversible photobleaching of β-carotene. No such effect is observed in the absence of the electron acceptors and it is dependent on the ability of the reaction centres to carry out charge separation. Flash absorption studies indicate that prior to the irreversible photobleaching, β-carotene is photooxidised by electron transfer to P680⁺. The rate of photobleaching of β-carotene is faster when SiMo is used as the acceptor and occurs both in the presence and absence of oxygen. However, with DBMIB present photobleaching is more clearly observed when oxygen is present. It is argued that when oxygen is absent, photoreduced DBMIB can rapidly rereduce P680⁺ by an electron transfer cycle …