Tyrosine phenoxyl radical (TyrO(.)) has been detected recently in a number of proteins by comparing experimentally observed electron paramagnetic resonance, UV resonance Raman, or Fourier transform IR vibrational spectra with the corresponding spectra for the organic phenoxyl radical (PhO(.)). Density-functional calculations are described to illustrate the strengths and limitations of the phenoxyl radical model for the structures, electronic spin densities, vibrational frequencies, and vibrational modes of TyrO(.). Both the PhO(.) and TyrO(.) radicals display substantial C=O double bond character, whereas distances within the carbon ring are intermediate between distances observed for the corresponding bonds of phenol and p-benzoquinone. The striking structural similarity between the two radicals appears despite the proximity of the CO2H and NH2 groups located gauche to the phenoxyl side chain of TyrO(.) in the amino acid radical’s most stable calculated gas-phase conformation.