The naphthalene radical cation generated from the corresponding neutral compound by UV- or X-irradiation on H-ZSM-5 zeolite and in frozen CFCl3 solution has been studied by EPR, ENDOR, and ESEEM spectroscopies. The g and ring proton hyperfine (hf) tensors have been determined and are identical in the two media, within experimental error. The isotropic hf couplings of the two sets of four equivalent protons are a(1) = 16.4 and a(2) = 4.7 MHz (0.587 and 0.167 mT, respectively). Semiempirical calculations of isotropic and dipolar hf couplings are found to be in good agreement with experiment. The hf tensors obtained for the radical cation of naphthalene-d(8) were analogous to those of the protonated compound, scaled by the gyromagnetic ratio, gamma(H)/gamma(D). The nuclear quadrupole interaction (nqi) tensors of the deuterium nuclei were also determined by ESEEM, being the only resolved method for the present nqi strength.