The thermodynamic parameters for the formation of the free radicals upon electron transfer quenching of the flavin triplet state (³FMN) by tryptophan and tyrosine, Δ_FRH and Δ_FRV, were obtained in aqueous solution by the application of laser-induced optoacoustic spectroscopy at various temperatures. The Δ_FRH and Δ_FRV values include the electron transfer and charge separation steps plus the protonation of the FMN anion radical and the deprotonation of the amino-acid cation radical. A linear correlation was found between the Δ_FRH and Δ_FRV values for each of the amino acids in phosphate buffers of [CH₃(CH₂)₃]₄N , Li , NH₄ , K and Cs . The compensation between Δ_FRH and Δ_FRV within the salt series, and the independent evaluation of the Gibbs energy for electron transfer Δ_ETG^o afforded the entropy change, Δ_FRS, for the reaction, different for the two amino acids. The values of Δ_FRH, Δ_FRV and Δ_FRS in each buffer are mainly determined by the changes in strength and probably number of hydrogen bonds between the reacting partners and water produced along all steps leading to the radicals FMNH^· and A^·. The Δ_FRV values linearly correlate with the tabulated entropy of organization of the water structure for the five cations, ΔS^o(cat). The entropy change upon formation of the free radicals, Δ_FRS, quantitatively correlated to the Δ_FRV value, drives the separation of the ion pair after the electron transfer reaction in the case of highly organizing cations. The ratio X = T Δ_FRS/Δ_FRV = (55 ± 9) kJ cm⁻³ for Trp as ³FMN quencher is smaller than X = (83 ± 9) kJ cm⁻³ for Tyr as quencher. These values are discussed in conjunction with the Marcus reorganization energy, as calculated from the Gibbs activation energy of the electron transfer process, which is independent of the salt present but different for each of the two quenchers.