The pH dependence of the decay rate of the P + Q − A state (P is the primary electron donor, and Q A , the first stable electron acceptor) was studied at cryogenic temperatures by the absorbance change technique. This study was done in aqueous solvent, in native reaction centers from Rhodopseudomonas viridis and in reaction centers from Rhodobacter sphaeroides in which the native Q A was replaced by the 1-amino-5-chloroanthraquinone. The previously reported biphasicity of the P + Q − A decays in both types of reaction centers (Sebban, P. and Wraight, C.A. (1989) Biochim. Biophys. Acta 974, 54–65; Sebban, P. (1988) Biochim. Biophys. Acta 936, 124–132) is also observed in aqueous buffer at low temperature. At variance to room temperature, where a marked pH dependence was previously observed, the relative distributions of the two components of the decay ( A fast and A slow ) remain constant at 90 K, in the pH range 5.5–11. A fast / A slow is equal to 20:80 and 40:60, in Rhodopseudomonas viridis and in modified Rhodobacter sphaeroides reaction centers, respectively. To further study the possible influence of rigidity of the protein environment on the above parameters, we have reconstituted the reaction centers from Rps. viridis in dimyristoylphosphatidylcholine and dielaidoylphosphatidylcholine liposomes. Below the phase transition temperatures of those lipids ( T c =23°C and 9.5°C, respectively), A slow dominates similarly to what occurs at low temperature. However, as the temperature is increased above T c , i.e. in the fluid phase of the lipid, A fast becomes greater than 50%. The same viscosity effect was observed in glycerol, where the A fast / A slow ratio drops from 1.22 at 35°C to 0.43 at −10°C, decreases slightly to about 0.25 at −30°C, and stays constant until 80K. Our data support the idea of two well-defined states of the reaction centers whose relative distribution may vary depending on the physical conditions, such as low temperature or viscosity, imposed by the medium. At 90 K, the rate constants of P + Q − A charge recombination in Rps. viridis reaction centers observed in aqueous buffer vary in the pH range 5.5–11.5 in a way that is reminiscent of what has previously been observed at room temperature, but with much lower amplitude of the variations. It is suggested that proton distribution present in the dark before cooling or/and proton motion and uptake by the protein at low temperatures modulate the free energy level of the P + Q − A state.