Thiols are a functional group commonly used for selective reactions in a biochemical setting because of their high nucleophilicity. Phosphorus nucleophiles can undergo some similar reactions to thiols, but remain underexploited in this setting. In this work we show that phosphine nucleophiles react cleanly and quickly with a dehydroalanine electrophile, itself generated from cysteine, to give a stable adduct in a peptide context. NMR reveals the product to be a phosphonium ion and indicates some backbone conformational constraint, possibly arising from transient carbonyl coordination. The reaction proceeded quickly, with a pseudo-first order rate constant of 0.126 min-1 at 1 mM peptide (80% conversion in 10 min), and with no detectable side products on the peptide. A broad peptide sequence scope and water-soluble phosphines with alkyl as well as aromatic groups were all shown to react efficiently. Phosphine addition proved to be efficient on nisin as a model Dha-containing biologically-derived peptide and on an mRNA-displayed peptide, as well as on TCEP-modified agarose for peptide capture from solution. This reaction thus presents a promising approach for modification of peptides for cargo attachment or altered physical properties in peptide discovery.