A series of silole (silacyclopentadiene)-thiophene copolymers, silole:thiophene 1:2, 1:3, and 1:4 copolymers, have been synthesized by palladium-catalyzed cross-coupling reactions. Silole-thiophene alternating 1:1 cooligomers (up to a seven-ring system) have also been prepared as models of their copolymers by nickel-promoted intramolecular cyclizations of the corresponding thiophene-1,6-heptadiyne cooligomers with hydrodisilanes. The UV-visible absorption data show that an increase in the silole content causes bathochromic shifts. In contrast, the conductivity tends to become higher with the higher thiophene content, the conductivity reaching up to 2.4 S cm(-1) upon doping with iodine. Theoretical studies on the electronic structures of silole-thiophene cooligomers suggest; that the unusual long absorptions observed for silole-thiophene mixed systems are mainly ascribed to the unique electronic structures of the silole rings.