1. The effects of lyophilization and the extraction of ubiquinone-10 on the kinetics of electron transport in Rhodopseudomonas capsulata Ala pho+ have been investigated. 2. Lyophilization reduced the amount of ferrocytochrome c2 photo-oxidized on a microsecond time scale following a single excitation. 3. Lyophilization increased the reactivity of the electron transfer components with redox mediators, particularly N-methyl phenazonium methosulphate (PMS). At a concentration of 1 µᴍ , PMS accelerated reaction center re-reduction, ferricytochrome c2 re-reduction and ferrocytochrome b50 oxidation. The cytochrome c2 re-reduction stimulated by PMS was antimycin A insensitive but the cytochrome b50 oxidation was partially antim ycin sensitive. 4. Removal of 25- 30 molecules of ubiquinone 10 per reaction center removed a secondary acceptor pool, had very little effect on the kinetics of ferricytochrome b50 reduction and ferricyto chrome c2 re-reduction, but markedly inhibited ferrocytochrome b50 oxidation. Ubiquinone extraction also caused an increased stimulation of ferrocytochrome b50 oxidation by PMS. 5. The involvement of tightly bound ubiquinone in cytochrome b reduction and in the cytochrome b-c2 oxido-reductase, and the role of semiquinone species is discussed.