Cl− has long been known as a micronutrient for oxygenic photosynthetic resulting from its role an essential cofactor for photosystem II (PSII). Evidence on the in vivo Cl− distribution in Spinacia oleracea leaves and chloroplasts shows that sufficient Cl− is present for the involvement in PSII function, as indicated by in vitro studies on, among other organisms, S. oleracea PsII. There is also sufficient Cl− to function, with K+, in parsing the H+ electrochemical potential difference (proton motive force) across the illuminated thylakoid membrane into electrical potential difference and pH difference components. However, recent in vitro work on PSII from S. oleracea shows that oxygen evolving complex (OEC) synthesis, and resynthesis after photodamage, requires significantly higher Cl− concentrations than would satisfy the function of assembled PSII O2 evolution of the synthesised PSII with the OEC. The low Cl− affinity of OEC (re-)assembly could be a component limiting the rate of OEC (re-)assembly.