In photosynthetic reaction centers from the purple bacterium Rps. viridis,  we have studied the backward electron transfer from the primary quinone acceptor, Q_A-, to the oxidized primary electron donor, P⁺ (a dimer of bacteriochlorophyll) under different conditions of salt concentration, lipid environment, viscosity and temperature. Biphasicity of this electron transfer process was recently demonstrated in isolated reaction centers (Sebban, P. & Wraight, C. A. (1989) Biochim. Biophys. Acta, 974, 54-65) and in chromatophores (Baciou, L., Rivas, E. and in chromatophores (Baciou, L., Rivas, E. and Sebban, P. (1990) Biochemistry, 29, 12, 2966-2976). It was suggested that this phenomenon arises from the existence of two populations or “conformational” states of the reaction centers, existing in vivo. We show here that, in addition to pH, the relative distribution of the two phases is sensitive to salt concentration, viscosity of the medium, degree of lipid rigidity, and to temperature. We have reconstituted the reaction centers into dimyristoylphosphatidylcholine and dielaidoylphosphatidylcholine liposomes. A marked effect of the lipid state is observed on the distribution of the relative amplitudes of the two states. Furthermore, freezing the protein at cryogenic temperature or by the surrounding lipids in their crystalline state, have similar effects of favoring the relative amplitude of the slow phase. This suggests that the equilibrium between the two reaction centers populations is sensitive to physical constraints imposed by the surrounding medium. Significant increase of the protonation pK’s associated with the P⁺Q_A- charge-separated state, was observed in 500 mM NaCl as compared to when no salt is present. This is consistent with the existence of a net positive charge provided by the protein on the cytoplasmic side of the reaction centers, close to the Q_A binding pocket.