Phylloquinone (PhQ) acts as the secondary electron acceptor in the reaction centre of Photosystem I. At room temperature the semiquinone anion is oxidized with complex multiphasic kinetics by electron transfer to the iron-sulphur cluster FX. The two principle phases of the oxidation kinetics are characterized by lifetimes of 20 ns and 250 ns. The 20-ns phase is associated primarily with the oxidation of PhQB, which is bound by the PsaB subunit, and the 250-ns phase is associated with oxidation of PhQA, which is bound by the PsaA subunit. The difference of about one order of magnitude between the two oxidation lifetimes can be explained by considering the difference in the driving force for oxidation of the PhQA (ΔG0 > 0) and PhQB (ΔG0 < 0) semiquinone forms. Such an energetic scenario also promotes a transient electron transfer from PhQA − to PhQB, with FX acting as an intermediary.