The kinetics of oxidation of Mn(II) with acylperoxyl and alkylperoxyl radicals were determined by laser flash photolysis utilizing a macrocyclic nickel complex as a kinetic probe. Radicals were generated photochemically from the appropriate ketones in the presence of molecular oxygen. In both acidic aqueous solutions and in 95% acetic acid, Mn(II) reacts with acylperoxyl radicals with k = (0.5-1.6) x 10(6) M-1 s(-1) and somewhat more slowly with alkylperoxyl radicals, k = (0.5-5) x 10(5) M-1 s(-1). Mn(III) rapidly oxidizes benzyl radicals, k = 2.3 x 10(8) M-1 s(-1) (glacial acetic acid) and 3.7 x 10(8) M-1 s(-1) (95% acetic acid). The value in 3.0 M aqueous perchloric acid is much smaller, 1 x 10(7) M-1 s(-1). The decarbonylation of benzoyl radicals in H2O has k = 1.2 x 10(6) s(-1).