AbstractThe rates of reduction of methionine sulphoxide to methionine by aqueous bisulphite solutions were measured by n.m.r. spectroscopy. The reaction was shown to follow overall second‐order kinetics, being first‐order with respect to methionine sulphoxide and first‐order with respect to bisulphite anion. The reduction was shown to have a strong pH dependence, reaching a maximum rate at pH 2.9. The pH‐rate profile has been related to the concentrations of bisulphite anion and protonated methionine sulphoxide and to the ionic state of methionine sulphoxide. A mechanism consistent with these results is proposed. An energy of activation of 14.1 kcal/mole was obtained for the reaction. These reductions are proposed to have important nutritional implications.