The energetics of the first stable charge separated state, P+QA- relative to that of P*QA was examined in isolated RC from Rhodobacter sphaeroides by delayed fluorescence. The temperature dependence of the delayed fluorescence indicates that the charge separation is a highly enthalpy-driven process (ΔH = – 818 ± 20 meV at pH 8) and the free energy gap between P*QA and P+QA- drops with increasing pH (40 ± 4 meV between pH 6 and 10). The pH-dependence of the free energy change of the P+QA- state runs parallel to the (integrated) net proton uptake due to the PQA/P+QA- redox change in a wide pH range and under different ionic conditions. Elevation of the ionic strength increases the delayed fluorescence intensity and decreases the (dark and light) pKA values as well as the light-induced ΔpKA changes of the protonatable groups of the protein. The observed dependence of the energetics of P+QA- on the concentration and composition of mobile ions is discussed in terms of binding and screening of protonatable groups and surface charges as dominant modes of electrostatic interaction between RC and salt.