The geometrical structure, fundamental vibrational frequencies, and isotropic hyperfine coupling constants of p-aminophenoxyl radical are investigated by means of electronic structure calculations. Neither Hartree-Fock nor full pi-space multiconfigurational self-consistent-field approaches are able to provide a satisfactory account of the experimentally observed vibrational frequencies. This is due mostly to not allowing the CN bond to participate sufficiently in the conjugation, thereby also allowing pyramidalization at the nitrogen site. Density functional methods, on the other hand, do give a reasonable description of the vibrational frequencies. Consideration of solvation effects by incorporation of four hydrogen-bonded water molecules, two at the oxygen site and two at the amino site, strengthens the CN bond and promotes planarity of the radical. With inclusion of solvent, the density functional vibrational frequencies-are further improved to come into very good agreement with experimental results in water. For the most part, hyperfine coupling constants are in satisfactory agreement with experiment. Vibrational averaging over large amplitude pyramidalization and torsional motions of the amino group atoms leads to only modest corrections to the hyperfine couplings and gives no support to an experimental report of significant temperature dependence of the amino hydrogen couplings in a presumably closely related derivative. However, averaging over these motions does give a good rationalization of the observed deuterium isotope shift in the amino:hydrogen coupling.