Chalcopyrite alloys with the composition (Cu(1−x)Agx)In(Se(1−z)Tez)2 have been synthesized and characterized. X-ray powder-diffraction measurements show well-defined single-phase material at each composition, verifying complete miscibility. Lattice parameters and differential-thermal-analyser-determined phase-transition temperatures vary smoothly throughout the composition range. Luminescence measurements have been obtained for a large range of excess carrier levels, and characteristics of the electronic band structure have been inferred by comparison of the observed spectra with spectra calculated using a recently developed model.At all compositions, the short, interband, luminescence decay time and the relatively high emission intensity identify the bandgap as direct. The results of the luminescence analysis also indicate that the band structure is dominated by a shallow acceptor ((Ea–Ev) = 10 to 40 meV), except in the AgInSe2 corner where the acceptor concentration drops low enough to allow a donor at (Ec–Ed) = 55 meV to take precedence. Alloying of the group VI anion causes a significant decrease in the bandgap, which drops to almost 100 meV below the minimum monochalcogen value. There is also a large increase in the concentration of the acceptor, which indicates that this acceptor is due to imperfections in the anion face-centered cubic (fcc) sublattice.