Recent studies of reaction centres from Rhodobacter sphaeroides (R-26), in which bacteriopheophytins a were replaced by plant pheophytins a, have shown that at low temperature the excited state of primary electron donor P* is converted to the state P+B-(A) (where B(A) is a bacteriochlorophyll a monomer in branch A) which has a long lifetime (about 600 ps [8]). This allows the direct measurement of the free energy difference between P* and P+B-(A) using the temperature dependence of the recombination fluorescence from P+B-(A). The data show that P+B-(A) is located below P* by 550+/-30 mV. Thus, the primary conversion of P* leads to the formation of P+B-(A) which is below P* in energy and is a real intermediate in electron transfer.