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The temperature dependences of the P870+Q−A → P870QA and P870+Q−B → P870QB recombination reactions were measured in reaction centers from Rhodopseudomonas sphaeroides. The data indicate that the P870+Q−B state decays by thermal repopulation of the P870+Q−A state, followed by recombination. ΔG° for the P870+Q−A → P870+Q−B reaction is −6.89 kJ · mol−1, while ΔH° = −14.45 kJ · mol−1 and −TΔS° = + 7.53 kJ · mol−1. The activation ethalpy, H‡, for the P870+Q−A Δ P870+Q−B reaction is +56.9 kJ · mol−1, while the activation entropy is near zero. The results permit an estimate of the shape of the potential energy curve for the P870+Q−A → P870+Q−B electron transfer reaction.