The thermodynamic of the known and very effective catalytic system, hydrogen peroxide (H2O2) and methyltrioxorhenium (MTO) is studied in different solvents using UV-Visible spectroscopic method. The thermodynamic parameters (ΔG, ΔH and ΔS) for two equilibrium reactions, MTO + H2O2 ⇌ A + H2O and A + H2O2 ⇌ B.H2O (A, [MeRe(O)2(O2)]; B.H2O, [MeRe(OH2)(O)(O2)2]), are determined. […]