Synthetic methods have been developed to generate the complete series of resonance-stabilized heterocyclic thia/selenazyl radicals 1a-4a. X-ray crystallographic studies confirm that all four radicals are isostructural, belonging to the tetragonal space group P (4) over bar2(1)m. The crystal structures consist of slipped pi-stack arrays of undimerized radicals packed about (4) over bar centers running along the z direction, an arrangement which gives rise to a complex lattice-wide network of close intermolecular E-2-E-2' contacts. Variable temperature conductivity (sigma) measurements reveal an increase in conductivity with increasing selenium content, particularly so When selenium occupies the E-2 position, with sigma(300 K) reaching a maximum (for E-1 = E-2 = Se) of 3.0 x 10(-4) S cm(-1). Thermal activation energies E-act follow a similar profile, decreasing with increasing selenium content along the series 1a (0.43 eV), 3a (0.31 eV), 2a (0.27 eV), 4a (0.19 eV). Variable temperature magnetic susceptibility measurements indicate that all four radicals exhibit S = 1/2 Curie-Weiss behavior over the temperature range 20-300 K. At lower temperatures, the three selenium-based radicals display magnetic ordering. Radical 3a, with selenium positioned at the E-1 site, undergoes a phase transition at 14 K to a weakly spin-canted (phi = 0.010 degrees) antiferromagnetic state. By contrast, radicals 2a and 4a, which both possess selenium in the E-2 position, order ferromagnetically, with Curie temperatures of T-c = 12.8 and 17.0 K, respectively. The coercive fields H-c at 2 K of 2a (250 Oe) and 4a (1370 Oe) are much larger than those seen in conventional light atom organic ferromagnets. The transport properties of the entire series 1a-4a are discussed in the light of Extended Huckel Theory band structure calculations.