Under the described reaction conditions for the synthesis of 125I-labelled thyroid hormones by the chloramine-T method, electrophilic substitution of the phenolic ring at positions 3' and 5' is accompanied by an exchange reaction. The proportions of mono- and diiodo-thyronine derivatives formed in this reaction depend on the specific activity of the 125I starting material. Doubly labelled molecules are far less stable than mono products, because the radioactive decay of one iodine atom is followed by destruction of the molecule itself, and the remaining iodine atom is released as I-. In the case of thyroxine with two equivalent substitution positions, there is always a high yield of doubly labelled molecules. By using a thyronine derivative containing a non-radioactive bromine atom in one of the two ortho positions of the 3,5-diiodothyronine molecule, it was possible to produce a "pseudo-T4" of high specific activity and excellent radiochemical stability by radioiodination of the second position. The immunological properties resemble those of T4. This compound may be useful as a tracer for the determination of free T4.