A comparative study of chalcopyrite-related Cu2In7Se11.5 and CuIn5Se8 crystals by spectroscopic ellipsometry is presented. Their complex dielectric function epsilon(omega) = epsilon(1) (omega) + i epsilon(2)(omega) has been determined in the 0.8-4.7 eV photon energy range. The spectral dependence of epsilon(1) (omega) and epsilon(2)(omega) as well as the complex refractive index, the absorption coefficient and the normal-incidence reflectivity for tetragonal Cu2In7Se11.5 and trigonal CuIn5Se8 crystals have been modelled using the Adachi's model for interband transitions. The results are in a good agreement with the experimental data over the entire range of photon energies. The model parameters, including the energies corresponding to the lowest direct gap (E-0) and higher critical points (E-1A, E-1B), have been determined using the simulated annealing algorithm. The results show that the tetragonal CuIn3Se5, the tetragonal Cu2In7Se11.5 and the trigonal CuIn5Se8 crystals belong to different phases of the Cu4In2nSe2+3n system, n = 6, n = 7 and n = 10, respectively. (C) 2010 Elsevier B.V. All rights reserved.