We report on the influence of Se substitution on the electronic band structure and thermoelectric properties (5-523 K) of the solid solution alpha-As2Te3-xSex (0 <= x <= 1.5). All of the polycrystalline compounds alpha-As2Te3-xSex crystallize isostructurally in the monoclinic space group C2/m (No. 12, Z = 4). Regardless of the Se content, chemical analyses performed by scanning electron microscopy and electron probe microanalysis indicate a good chemical homogeneity, with only minute amounts of secondary phases for some compositions. In agreement with electronic band structure calculations, neutron powder diffraction suggests that Se does not randomly substitute for Te but exhibits a site preference. These theoretical calculations further predict a monotonic increase in the band gap energy with the Se content, which is confirmed experimentally by absorption spectroscopy measurements. Increasing x up to x = 1.5 leaves unchanged both the p -type character and semiconducting nature of alpha-As2Te3. The electrical resistivity and thermopower gradually increase with x as a result of the progressive increase in the band gap energy. Despite the fact that alpha-As2Te3 exhibits very low lattice thermal conductivity k(1), the substitution of Se for Te further lowers xi, to 0.35 W m(-1) K-1 at 300 K. The compositional dependence of the lattice thermal conductivity closely follows classical models of phonon alloy scattering, indicating that this decrease is due to enhanced point defect scattering.