The development of an efficient, catalytically active, biomimetic model for cytochrome P-450 enzymes has been an area of intense research activity. Ruthenium porphyrin complexes have been the center of this research and have successfully been utilized, as catalysts, in major oxidation reactions such as the hydroxylation of alkanes, the epoxidation of alkenes and aromatic rings, and the N-oxidation of amines, among others. In this project, the kinetics of two-electron sulfoxidation of para-substituted phenyl methyl sulfides to the corresponding sulfoxides with well-characterized trans-dioxoruthenium(VI) porphyrin complexes were studied by rapid stopped-flow spectroscopy. The substituent effect in sulfides and in dioxoruthenium(VI) complexes were also kinetically investigated. The low-reactive trans-dioxoruthenium(VI) porphyrin complexes (3a-b) were synthesized from the oxidation of their carbonylruthenium(II) porphyrin precursors with m-chloroperoxybenzoic acid (m-CPBA) and characterized spectroscopically by 1H-NMR, IR, and UV-vis. The low-reactivity of these complexes makes them suitable for kinetic studies. The sulfoxidation with the trans-dioxoruthenium(VI) species followed a pseudo-first order kinetic decay from RuVI to RuIV species with no accumulation of intermediates. The reactivity order in the series of dioxoruthenium(VI) complexes follows 3b > 3a >3c, which is consistent with expectations based on the electrophilic nature of high-valent metal-oxo species. Steric effect of the substituents on the complexes also affected the reactivity order. The kinetic results revealed that the sulfoxidation reaction with these well-characterized dioxoruthenium(VI) complexes is 3 – 4 orders of magnitude faster than the epoxidation reaction with the same complexes under similar conditions.