Cytochrome c peroxidase (CcP) and horseradish peroxidase (HRP), when combined with a stoichiometric amount of H2O2, form stable compounds I which are known as FeIV Ro and FeIV o pi + structures, respectively. These compounds were assayed in the catalysis of thyroid hormone synthesis and the iodination reaction. As previously shown for the lactoperoxidase FeIV Ro compound, the CcP FeIV Ro compound was involved in the coupling and not in the iodination reactions. In contrast, the HRP FeIV o pi + compound catalyzed both iodination and hormone formation. The possible role of the different peroxidase-H2O2 compounds in the two sequential reactions, thyroglobulin iodination and thyroid hormone formation, is discussed.