The irradiation of 4-hydroxy-2,2,6,6-tetramethylpiperidine (I) by x-rays produces radicals at both the nitrogen and oxygen atoms in the molecule. With short irradiation times the nitrogen radical is produced but with longer exposures the oxygen radical is predominant. This occurs in the powder sample, whereas in single crystals only the oxygen radical is observed. Quantum chemical semi-empirical methods predict the formation of both radicals with preference, from the energetic point of view, for the nitrogen radical. In the present work a detailed study of the oxygen radical both in powder and single crystal forms of I was performed, allowing the determination of the hyperfine interactions with the protons in the ring. The unpaired electron interacts with one of the protons at carbons C-3 and C-5 producing a triplet (isotropic value 3.89 mT); further interaction with the proton at carbon C-4 splits each line into doublets (isotropic value 2.07 mT). Finally, interaction with the remaining protons at C-3 and C-5 splits each line into new triplets (isotropic value 1.13 mT). In order to fit the angular variations in the single crystal and to obtain the spectroscopic tensors, measurements were performed at Q-band where the resolution of lines was appropriate. The g-tensor is virtually isotropic and the hyperfine interactions are also dominated by the isotropic contribution. The g-values are g , = 2.0035, gpr = 2.0033 and g,, = 0.0022, which are consistent with those observed for nitroxide radicals. The calculations of the HOMOS for the radicals showed a great contribution of p,-type orbitals at nitrogen or oxygen atoms and the additional orbitals involved correspond qualitatively to those observed from the ESR data. Hence theoretical calculations qualitatively describe the experimental ESR results. The calculated spin densities are not in quantitative agreement with the proton splittings observed from ESR data. The results also suggest that the molecule of I is fairly rigid, favouring the localization of the oxygen-centred radical around the C-4-C-3 and C-4-C-5 bonds.