The compounds Tl9BiTe6 (TBT) and Tl9BiSe6 (TBS) crystallize in the tetragonal space group I4/mcm. Tl9BiTe6 has a thermopower of 185 μV/K and an electrical resistivity of 5.5 mΩcm at 300K, resulting in a power factor of S
2/ρ = 0.6 mW/mK2. Compared to Bi2Te3 which is the state of the art material at this temperature this is about a factor of 7 lower. At 300 K TBS has a thermopower of 750 μV/K but a high resistivity of 130 Ωcm. To optimize the thermoelectric properties of TBT solid solutions have been formed with TBS. The resistivities and have been measured on Tl9BiTe1-xSex with x = 0.05, 0.08, 0.2 and 0.5. In addition to the electrical properties the lattice constants have been measured by X-ray diffraction. The dependence of the lattice constants on the Te/Se ratio clearly deviates from Vegard's law. Different affinities of Te and Se towards the two chalcogenide sites in the crystal can explain this behavior.