We describe a new method for the separation, identification and quantitation of iodotyrosines and iodothyronines [3-monoiodo-L-tyrosine (MIT), 3,5-diiodo-L-tyrosine (DIT), L-thyronine (T0), 3,5-diiodo-L-thyronine (T2), 3,5,3'-triiodo-L-thyronine (T3), reverse 3,3',5'-triiodo-L-thyronine (rT3) and 3,3',5,5'-tetraiodo-L-thyronine (T4)]. Reversed-phase high-performance liquid chromatography (RP-HPLC) was performed on a Nucleosil C8 column with photodiode-array UV-Vis detection. A clearly defined elution profile was obtained of each iodoamino acid (iodotyrosines and iodothyronines) using a linear gradient from 20 to 80% phase B (90% acetonitrile, 10% water, 0.1% TFA), phase A (water, 0.1% TFA, pH 2.0) eluted over 40 min. Iodoamino acid composition was determined, taking into account retention times and spectral characteristics. Thyroid protein samples were digested enzymatically and the complex mixture of IAA was then injected onto the RP-HPLC system. A photodiode-array detector with a dynamic range in the UV-Vis region was used in the HPLC system to monitor the absorbance at different wavelengths continuously, collecting data which were compared with standard samples. Each IAA was quantitated using linear calibration curves obtained at 280 nm. This method allowed identification and quantitation of iodoamino acids from diverse sources in the range 2-500 ng, avoiding the need to radiolabel samples. The technique was tested with in vitro iodinated and non-iodinated human thyroglobulin and the recoveries ranged from 84 to 91%.