Protein sulfenic acids are reactive, reversibly oxidized cysteinyl residues with roles in redox catalysis and regulation. Detection and quantification of these species in proteins is accomplished through chemical modification by reagents such as 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD chloride), 2-nitro-5-thiobenzoate (TNB), dimedone, or derivatives of dimedone, followed by UV-visible spectroscopy or mass spectrometric analysis.