Optical pump-probe spectroscopy in the nanosecond–microsecond timescale has been used to study the electron transfer reactions taking place within the Photosystem I reaction center of intact Chlamydomonas reinhardtii cells. The biphasic kinetics of phylloquinone (PhQ) reoxidation were investigated in double mutants that combine a mutation (PsaA-Y696F) near the primary acceptor chlorophyll, ec3A, with those near PhQA (PsaA-S692A, PsaA-W697F). The PsaA-S692A and PsaA-W697F mutations selectively lengthened the 200 ns lifetime component observed in the wild-type (WT). The ?20 ns component was unaltered in the single mutant, both in terms of lifetime and relative amplitude. However, both double mutants possessed a ?20 ns component (PhQB reoxidation) with increased amplitude compared with the WT and the individual PhQA mutants. The component assigned to PhQA reoxidation was slowed, like the individual PhQA mutants, and of lower amplitude, as observed in the single ec3A mutant. Hence, the effects of these mutations are almost entirely additive, providing strong support for the previously proposed bidirectional electron transfer model, which attributes the ?20 and ?200 ns phases to reoxidation of PhQB or PhQA, respectively. Moreover, in all the mutants investigated, it was also possible to observe an intermediate (∼180 ns) component, as previously reported for mutants of the PhQA binding pocket (Biochim. Biophys. Acta [2006] 1757, 1529–1538), which we have tentatively attributed to forward electron transfer between the iron–sulfur clusters FX and FA/B.