The reactions of 1,4-phenylenebis[N,N,N′-tris(trimethylsilyl)amidine] with sulfur and selenium dichlorides yield, respectively, the l,4-phenylenebis(dithiadiazolium) and bis(diselenadiazolium) dichlorides l,4-[(E2N2C)C6H4(CN2E2)]Cl2 (E = S, Se). Reduction of these materials with triphenylantimony affords the corresponding diradical species 1,4-[(E2N2C)C6H4(CN2E2)], which can be purified by high vacuum sublimation. The crystal structures of 1,4-[(E2N2C)C6H4(CN2E2)] (E = S, Se) are both monoclinic, space group P21/n. The crystal packing consists of sheets of interleaved columns of weakly associated diradical units (dimers); the mean intradimer separation is 3.34/3.37 Å (E = S/Se), and the mean interdimer separation is 3.68/3.71 Å (E = S/Se). The sulfur compound is an insulator, but the selenium material has a room temperature pressed pellet resistivity of about 100 fi cm. Magnetic susceptibility measurements show the solids to be predominantly diamagnetic with variable amounts of paramagnetic defects in the lattice. While the exact mechanism of conduction remains to be established, these systems represent the first structurally characterized conducting materials constructed from neutral molecular radicals. Extended Hiickel band structure calculations reveal band gaps of 0.84/0.69 eV (E = S, Se) for the crystalline solids. The calculated band dispersions show a high degree of three-dimensionality; to our knowledge these are the most isotropic organic molecular solids yet to be reported.