Diruthenium(II,III) carboxylates Ru-2(esp)(2)Cl (1a), [Ru-2(esp)(2)(H2O)(2)]BF4 (1b), and Ru-2(OAc)(4)Cl (2) efficiently catalyze the oxygenation of organic sulfides. As noted in a previous work, 1a is active in oxygenation of organic sulfides with tert-butyl hydroperoxide (TBHP) in CH3CN. Reported herein in detail is the oxygenation activity of 1a, 1b, and 2, with the latter being highly selective in oxo-transfer to organic sulfides using TBHP under ambient conditions. Solvent-free oxidation reactions were achieved through dissolving 1a or 1b directly into the substrate with 2 equiv of TBHP, yielding TOP up to 2056 h(-1) with 1b. Also examined are the rate dependence on both catalyst and oxidant concentration for reactions with catalysts 1a and 2. Ru-2(OAc)(4)Cl may be kinetically saturated with TBHP; however, Ru-2(esp)(2)Cl does not display saturation kinetics. By use of a series of para-substituted thioanisoles, linear free-energy relationships were established for both 1a and 2, where the reactivity constants (rho) are negative and that of 1a is about half that of 2. Given these reactivity data, two plausible reaction pathways were suggested. Density functional theory (DFT) calculation for the model compound Ru-2(OAc)(4)Cl.TBHP, with TBHP on the open axial site, revealed elongation of the O-O bond of TBHP upon coordination.