Disulfide bonds can be cleaved at an alkaline pH by treating a protein with excess of a reagent disulfide in the presence of catalytic amounts of thiol. The cleavage products are stable and can be isolated; they contain the mixed disulfide between the reagent and the exposed thiol groups of the protein. The extent of cleavage is readily controlled by the pH of the reaction, temperature, and the addition of urea. Disulfide bonds cleaved by the reaction can be re-formed by exposing the mixed disulfide of the protein to catalytic amounts of thiol. Specific side chains can be added on to the thiol groups in native proteins by treatment with a reagent disulfide alone.