The results of an ab-initio SCF calculation of the hyperfine coupling parameters in the nπ* lowest triplet state of p-benzoquinone are reported and discussed. New results of a ring carbon-13 and oxygen-17 ENDOR study on the lowest triplet state of p-benzoquinone are also reported. Excellent agreement with experiment is obtained for the anisotropic hyperfine interaction constants which implies that the SCF description of the singly occupied orbitals is basically correct. The basic picture of the lowest triplet excited state spin-density distribution that is obtained from theory and experiment is, that the excited π-electron is delocalized over the molecule, while the n-electron is largely (≈75%) confined to the oxygen atoms. The effect of the delocalization of the n-electron however is clearly evident from the experiments.