Abstract: Peptides containing a disulfide bond between two cysteine residues are formed by an oxidation process. The oxidation step is accomplished under conditions whereby the sulfhydryl concentration in a reaction mixture is maintained at substantially zero during the reaction. In the process, an aqueous solution of a peptide containing at least one pair of cysteine moities is added incrementally to a buffered solution containing an oxidizing agent. The period between addition of the increments of the peptide solution is sufficient that the reaction to form the disulfide bond occurs substantially instantaneously. The increments are of such size that the sulfhydryl concentration during the reaction remains substantially constant and at a level of substantially zero, equivalent to infinite dilution.