A new low band gap conducting polymer, poly(2-decylthieno[3,4-b] thiophene-4,6-diyl) (5), predicted to have nearly equi-energetic aromatic and quinonoidal forms, was synthesized by a multistep sequence involving FeCl3 polymerization of 2-decylthieno[3,4-b]thiophene (7) and studied. A film of 5 showed a vis-NIR spectral lambda (max) = 925 nm, a band edge band gap of 0.92 eV, and an FeCl3-doped conductivity of 3.1 x 10(-3) S cm(-1). A CHCl3 solution showed lambda (max) = 739 nm and a band-edge band gap of 0.98 eV. GPC (polystyrene standards) gave a number-average molecular weight, (M) over bar (n), of 52 000 with a polydispersity (M) over bar (w)/(M) over bar (n) of 1.7. The band gap of 0.92 eV makes 5 one of the lowest band gap organic polymers known.