We study the detailed temperature and composition dependence of the resistivity, ρ(T), and thermopower, S(T), for a series of layered bismuth chalcogenides Bi2Te3−xSex, and report the stoichiometry dependence of the optical band gap. In the resistivity of the most compensated member, Bi2Te2.1Se0.9, we find a low-temperature plateau whose onset temperature correlates with the high-temperature activation energy. For the whole series S(T) can be described by a simple model for an extrinsic semiconductor. By substituting Se for Te, the Fermi level is tuned from the valence band into the conduction band. The maximum values of S(T), bulk band gap as well the activation energy in the resistivity are found for x≈0.9.