The reversible electron transfer from donor to excited molecule (acceptor of electron) is shown to be the irreversible energy quenching, if it is completed by subsequent irreversible recombination radical-ions which are produced. The Stern-Volmer constant of fluorescence as well as the Markovian rate constant of triplet quenching are calculated analytically, assuming the electron transfer is contact. The multiple Rehm-Weller effect is shown to be peculiar to both constants.