The kinetics and thermodynamics of Ru-III-EDTA-ascorbate-O-2 system catalyzed epoxidation of cis-cyclooctene, styrene and trans-stilbene were investigated in mixed aqueous acidic (pH=1.5-2.5) solvent of water : dioxane (1:2) v/v at mu=0.1 M KNO3 in the temperature range 288-313 K. The kinetics of the epoxidation of olefins studied at each particular temperature as a function of variants gave first order dependence on concentration of the catalyst, ascorbate, dioxygen and olefin and an inverse first order in hydrogen ion concentration. The activation parameters corresponding to rate determining steps and the thermodynamic parameters corresponding to equilibrium steps of all kinetic intermediates, spectrophotometrical ly and kinetically characterized, were determined. A comparison of the activation parameters and quantitative yield of epoxides formed in presence (monooxygenase) and absence (dioxygenase) of ascorbic acid indicated that monooxygenase system was more efficient for epoxidation. Both, enthalpy Delta H-# and entropy Delta S-# Of activation were found to be highly reduced in presence of ascorbate. Linear free energy relationship between Delta H-# and Delta S-# and between rate and equilibrium constants were also obtained.