The cationic complex [(pymox-Me(2))RuCl(2)](+)BF(4)(-) was found to be a highly effective catalyst for the C-H bond oxidation reaction of arylalkanes in water. For example, the treatment of ethylbenzene (1.0 mmol) with t-BuOOH (3.0 mmol) and 1.0 mol % of the Ru catalyst in water (3 mL) cleanly produced PhCOCH(3) at room temperature. Both a large kinetic isotope effect (k(H)/k(D) = 14) and a relatively large Hammett value (rho = -1.1) suggest a solvent-caged oxygen rebounding mechanism via a Ru(IV)-oxo intermediate species.