AbstractIodinated peptides or proteins (125I,131I) are used as tracers for radioimmono assays and for detection in biological systems. The labelling takes place on the aromatic side chains.However, when disulfide bridges are present, iodination must be carried out under conditions preventing their degradation. The rate of iodination of L‐tyrosine, L‐histidine and L‐histidine containing peptides by iodine monochloride (ICI) are studied by spectroscopy. Free hitidine increases the hydrolysis rate of aqueous solution of ICI. The interactions between ICI and some simple disulfide are investigated by circular dichroism measurements. These reactions are presented as models for degradation of disulfide bridges in peptide or protein during the iodination pr