The reactivity of the hydroperoxo complex [Co(CN)(5)OOH](3-) has been studied in aqueous solution. The complex undergoes acid-catalyzed aquation (k = 1.89(5) x 10(-2) s(-1), pK(a) = 5.21(4), T = 20 degrees C, I = 0.1 M). Assuming an Id mechanism, this allows the relative affinity for Co(III) to be deduced as H2O2 < H2O < HO2- and implies H2O2 to be a very weak ligand. At neutral pH the hydroperoxo complex effects efficient oxygen atom transfer to L-methionine to give an intermediate identified as [Co(CN)(5)(L-methionine S-oxide)](2-), which then dissociates to [Co(CN)(5)OH2](2-) and L-methionine S-oxide. The reaction is acid catalyzed and is proposed to take place via nucleophilic attack of sulfur on the proximal oxygen of the hydroperoxo ligand with concerted loss of water. The significance of these results for the interaction of hydrogen peroxide with labile metal ions is discussed.