All three disulphide bonds in insulin are broken by the combined action of sulphite and oxygen, the reaction proceeding at a reasonable rate at room temperature, pH 8-10. This " oxidative sulphitolysis " (RSSR'+RSSO< +RISSO<) is strongly catalysed by metal ions, particularly Cu(11). For a 1% solution of insulin at pH 10 containing 2x 10-4iu CU(II), reaction is complete in about 24 hr at 20 OC, and acidification then gives a precipitate of the S-sulpho B-chain, the S-sulpho A-chain remaining in solution. Catalysis of the air-oxidative sulphitolysis of cystine to S-sulphocysteine (Clarke 1932 ; Kolthoff and Stricks 1951a, 1951b) is confirmed, CU(II), M~(II), CO(IIL), and F~(III) being most effective in that order. In these reactions the binding of metal ions by intermediate thiols prevents or retards the usual metal-catalysed air oxidation of sulphite to sulphate, so that a large excess of sulphite is not required. When a mixture of insulin and sulphite is oxidized with CU(II) with exclusion of oxygen, two end-points are readily detected (cf. cystine ; Kolthoff and Stricks 1951a, 1951b), corresponding to the reactions In addition there is evidence for the formation, in the early stages, of cupric mercaptide according to the equation Cupric sulphitolysis can also be carried out in two stages by &st titrating the insulin with CU(I) to the end-point of the reaction and then with CU(II) to complete the reaction according to RSCU+SO;- +2Cu2++RSS0c +3Cu+.