Uncharged, water-soluble nitroxyl free radicals are shown by laser flash photolysis to be effective reductants of the Ru(bpy)(3)(3+) (bpy = 2,2’-bipyridine) that is produced via photosensitization at the surfaces of colloidal semiconductor particles composed of antimony-doped tin dioxide. The bimolecular rate constant for the reduction of Ru(bpy)(3)(3+) by the 3-carbamoyl-PROXYL radical in the colloidal suspension, (4.9 +/- 0.2) x 10(8) M(-1) s(-1), is about one-third of the rate constant for the corresponding reaction in homogeneous aqueous solution, (1.4 +/- 0.1) x 10(9) M(-1) s(-1). The high quantum efficiency for production of Ru(bpy)(3)(3+), 0.93 +/- 0.01, together with the fact that the oxidized nitroxyl radicals are widely used as oxidants in organic syntheses, makes such systems of interest for effecting photocatalytic oxidation reactions.