The Stern−Volmer constant of fluorescence quenching by reversible intermolecular charge transfer is obtained by means of integral encounter theory. The latter provides the first non-Markovian description of the phenomenon which accounts for the reversibility of excited-state ionization. The forward and backward electron transfers (bimolecular and geminate) are specified by the position-dependent rates of ionization and recombination. Assuming that the conventional free energy gap law is inherent to all of them, a reasonable explanation is given for the famous Rehm and Weller free energy dependence of the Stern−Volmer constant. It requires the production of ions in excited states when forward electron transfer is highly exergonic and implies that the charge recombination occurs not only to the ground but also to the excited triplet state. It is assumed that spin conversion in the radical ion pairs is faster than the geminate recombination.