EPR spectroscopy of single crystals of tetraethylammonium 2,3,5,6-tetracyano-p-benzoquinonide, ([Et(4)N](+))(2)[Q](2-)(2), at temperatures near ambient indicates the presence of an electronic triplet species (S = 1) exhibiting fine-structure splitting. At 380 K the g- and spin-spin interaction matrices were established from measurements of the fine-structure splitting as a function of angle in three orthogonal crystal planes of crystallographically oriented single crystals : g = (2.0020, 2.0079, 2.0048); D = (-379.6, 243.7, 141.2 MHz). The g(2)-tensor is aligned within a few degrees of the local symmetry elements of the constituent [Q](-) anions of the crystal, i.e., perpendicular to the ring, along 0-0, and across the ring. Comparison of the eigenvectors of D with crystal directions shows that the triplet spectrum is not due to intradimer electronic interaction, i.e., interaction associated with the constituents of the [Q](2-)(2) dimer anion, but rather to interdimer interactions within the anion stack. The change in D as a function of temperature is explained in terms of thermal expansion of the crystal. From the temperature dependence of the spectral intensity it is established that the radical pair has a singlet ground state which lies 3075 +/- 42 cm(-1) (8.79 +/- 0.12 kcal/mol; 0.381 +/- 0.005 eV) below the observed triplet.