For reaction centers of photosynthetic bacteria reconstituted with low-potential quinones in the QA site, the state P(+)Q(A)(-) formed by light activation decays to the ground state via a thermally activated route through the P+H- state. The rate of charge recombination by this thermal pathway is proportional to the equilibrium constant between P(+)Q(A)(-) and P+H-. Thus, the free energy difference between P(+)Q(A)(-) and P+H- can be determined by measuring the charge recombination rate via the uphill route. The enthalpy and entropy change of the reaction can then be deduced from the temperature dependence of the charge recombination kinetics. The free energy, entropy, and enthalpy changes between P(+)Q(A)(-) and P+H- were determined at temperatures from 40 to 318 K for several low-potential quinones. From 200 K to room temperature, Delta H degrees approximate to Delta G degrees, so the entropy changes are small. However, in the temperature range 80-200 K, a significant entropy change is observed, and the free energy becomes strongly temperature-dependent. The newly formed P(+)Q(A)(-) State lives for milliseconds. On this time scale at low temperature, the P(+)Q(A)(-) State appears to be trapped prior to charge recombination in a state similar to 200 meV (10 K) higher in free energy than the relaxed form found at room temperature.