Here we show that a four-membered oxygen ring – oxetane – can be readily grafted into native peptides and proteins through site-selective bis-alkylation of cysteine residues present as disulfides under mild and biocompatible conditions. The selective installation of the oxetane graft enhances stability and activity, as demonstrated for a range of biologically relevant cyclic peptides, including somatostatin, proteins and antibodies, such as a Fab arm of the antibody Herceptin® and a designed antibody DesAb-A against the human Amyloid- peptide. Markedly, oxetane grafting of the genetically detoxified diphtheria toxin CRM197 improves significantly the immunogenicity of this protein in mice, which illustrates the general utility of this strategy to modulate the stability and biological activity of therapeutic proteins containing disulfides in their structures.