The phase diagram of the Cu(In1−xGax)5Se8 (0 ≤ x ≤ 1) alloy system, prepared by direct fusion of the stoichiometric mixture of the elements, has been established by X-ray diffraction (XRD) and differential thermal analysis (DTA). The presence of a tetragonal structure at room temperature is found in the composition range 0.4 ≤ x ≤ 1.0. The reason for the reported tetragonal and hexagonal structures of CuIn5Se8 an also their co-existence below x < 0.4 observed in the present work is explained. In the composition range 0 ≤ x ≤ 1, the unit cell lattice parameters a and c of the tetragonal phase vary from 5.73–5.48 Å and 10.90–11.56 Å, respectively. From the absorption coefficient spectra, the band gap energy is found to vary from about 1.17 to 1.85 eV, with a downward bowing parameter b2 ≈ 0.21 eV. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)