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Prolonged illumination of the isolated Photosystem II reaction centre under aerobic conditions causes a selective photodestruction of chlorophyll which absorbs maximally at 680 nm. Concomitant with this effect is a loss of photochemical activity. When oxygen is absent, the reaction centre is no longer damaged by illumination. Protection of the 680 nm absorbing chlorophyll against photodamage and maintenance of photochemical activity can also be achieved if silicomolybdate (SiMo) is present as an electron acceptor, although in this case there is an irreversible bleaching at 670 nm, whether oxygen is present or not. We therefore suggest that there are two chlorophyll species in the reaction centre absorbing at 670 nm and 680 nm, respectively. The latter we attribute to the primary donor P680 and the former to an accessory chlorophyll. It seems highly likely that in the absence of SiMo but under aerobic conditions …