The electronic structure of tetrathiafulvalene (TTF) and its three conformational isomers, 2,5-dimethylene-1,3-dithiolo[4,5-d] -1.3-dithiol (DDD), 1,4,5,8-tetrathianaphthalene (TTN), and bis(4-methylene-1,3-dithietan-2-ylidene) (BMDY), are studied using ab initio and PRDDO molecular orbital calculations and extended Huckel band calculations. The one-dimensional expansion of DDD is identical to that of TTF. TTN has two conformations, corresponding to a boat and chair form. Our calculations show that TTF is the most stable structure among the monomers studied. The stability of the polymers arising from these monomers is estimated by oligomer calculations extrapolated to infinite chain length. The bandwidths of their highest occupied band are rather narrow (0.54-1.03 eV); however, the three highest valence bands of each isomer are separated from each other by 0.1-0.4 eV. The calculated band gaps (2.7-2.9 eV) of all the isomers are larger than that of polythiophene (1.75 eV), whereas the ionization potentials (9.7-10.1 eV) are smaller than that of polythiophene (11.0 eV). Thus, it should be easier to make a p-type conductor by doping the systems with an electron-accepting molecule.