We have measured the equilibrium constant for the reaction by which the plastoquinol: plastocyanin oxidoreductase (bf complex) oxidizes plastoquinol. By destroying or removing plastocyanin from spinach thylakoids, the terminal electron acceptor for oxidation of plastoquinol was changed from the oxidized primary donor of photosysten 1 (P700+) to oxidized cytochrome f (cyt f+). Because cytochrome f has a lower mid-point redox potential than P700, the equilibrium constant for oxidation of plastoquinol was lowered by this treatment. Chloroplasts were poised at high ambient redox potential (Eh) (about +450 mV), to oxidize the cytochromes of the bf complex, and then excited by actinic flashes so that photosystem 2 (PS2) produced excess PQH2. After such treatment, cytochromes b and f became partially reduced following flash excitation. Cytochrome f reduction occurred in two distinct phases: a rapid phase with t1/2 = about 6-10 ms, and a slow phase with t1/2> 100 ms. The rapid reduction phase corresponded well in kinetics and extent to the rapid reduction phase of cytochrome b (cyt b). The slow phase of cyt f reduction was linked to re-oxidation of cyt b, with complex kinetics. From these results, and estimation of the concentration of plastoquinone (PQ) and plastoquinol (PQH2), we conclude that the concerted reduction of cyt b and f by PQH2 at the PQH2 oxidizing site (Qo-site) reached its local thermodynamic equilibrium at the end of the fast phase of cyt f reduction. At this point, there was not sufficient driving force in the PQ/PQH2 couple to further reduce both cyt f and cyt b, and cyt f reduction could occur only as cyt b became re-oxidized through the PQ-reductase site of the bf complex. From these data, we estimate that, at pH 7.3, the equilibrium constant for the concerted reduction of cyt b and cyt f by the PQ pool was approximately 10, which is consistent with the equilibrium constant calculated from measured values for the midpoint potentials of the redox components. By repeating these experiments at different values of pH, we have shown that the equilibrium constant is pH dependent, and follows the value expected from the measured pH-dependencies of midpoint potentials for the redox components. Our hypothesis is further supported by experiments in which the re-oxidation of cyt b was strongly inhibited, and the slow phase of cyt f reduction was greatly slowed, by addition of MOA-stilbene, or the re-oxidation of cyt b was increased by addition of benzoquinone (BQ), when the rate of the slow phase of cyt f reduction was also increased. The BQ-induced effects were largely reversed by addition of MOA-stilbene, indicating that the oxidation of cyt b by BQ is most likely catalyzed by the Qi-site. We conclude that the primary pathway for plastoquinol oxidation at the Qo-site is via a classical oxidant-induced reduction mechanism and does not occur via a semiquinone (SQ) cycle or any other mechanism that allows reduction of the high potential chain without a concerted reduction of the low potential chain.