The separation of low molecular weight compounds labelled with125I has gained importance with the advent of radioimmunoassay (RIA) in which overwhelmingly125I-labelled analytes are used as tracers. The chloramine T labelling method proved to be the most suitable procedure to introduce radioiodine atom via aromatic electrophilic substitution either in the thyronine molecule or in the tyrosine methyl ester (TME) side chain coupled as a prosthetic group to different compounds which do not possess phenolic hydroxyl groups.