We have analyzed the rate of electron transfer between the tetrahemic cytochrome and the primary electron donor in isolated reaction centers of Roseobacter denitrificans as a function of the ambient redox potential. Three different phases are observed: a slow phase (half-time > ms), and two fast phases with half-times of 5 µs and 380 ns. The slow phase is present at high redox potential, it corresponds to the kinetics of charge recombination between the photo-oxidized primary electron acceptor P+ and the reduced primary acceptor (QA-). The 5 µs phase titrates with the reduction of the highest potential heme (HP1). This phase corresponds to the electron transfer between heme HP1 and P+. At redox potentials where the second high potential heme HP2 becomes reduced, the 5 µs phase disappears and is replaced by the 380 ns phase, which is therefore related to the electron transfer between the high potential heme HP2 and P+. To explain the large difference in the rate of oxidation of HP1 and HP2 we propose a tentative model where the heme HP2 is closest to P.