The electron paramagnetic resonance of x-irradiated (∼10 krad.) tyrosine, thyronine and their iodine-derivatives in the form of compressed pellets in vacuum-sealed quartz tubes have been measured between 100 K and 300 K at ∼9 GHz. The EPR spectra of tyrosine, 3–5 diiodo thyronine and thyronine, in the order of decreasing line-width, are spectrally similar singlets with g = 2.004 or 2.005 and hence indicate that the electron spin density is trapped mainly in the ring from which the hydroxyl proton is lost. The samples having iodine on the same ring as the OH group show a rather large anisotropy in g ≊ 2.04 presumably due to the presence of small spin density on the iodine(s).