The selenium analogues of tetrathiafulvalene (TTF), tetraselenafulvalene (TSeF) and diselenadithiafulvalene (DSeDTF), have been synthesized by trimethyl phosphite coupling of 1,3-diselenole-2-selone and l,3-thiaselenole-2-selone, respectively. TTF, TSeF, and DSeDTF form isostructural, metallic charge-transfer salts with tetracyano-p-quinodimethane (TCNQ). TSeF-TCNQ has a slightly higher conductivity and a metal-insulator transition at lower temperature compared to TTF-TCNQ. The isostructurality of TSeF-TCNQ and TTF-TCNQ permits the formation of solid solutions TSeFxTTF1-xTCNQ where x can be varied from 0 to 1. Solid solution compositions of single crystals, grown by slow cooling from saturated CH3CN solutions, were determined by elemental analysis, x-ray analysis, and electron microprobe, and found to be homogeneous. Four-probe dc conductivity measurements as a function of temperature and powder x-ray measurements of the unit cell constants were carried out over the entire solid solution range. © 1977, American Chemical Society. All rights reserved.