We have determined the complex dielectric tensor components of the chalcopyrite semiconductor CuAlSe ₂ in the energy range between 1.4 and 5.2 eV, at room temperature, using spectroscopic ellipsometry. We present results obtained on two single crystals grown by the traveling-heater method using In solvent. Values of refractive indices n, extinction coefficients k and normal-incidence reflectivity R in the two independent polarizations are reported. The structures observed in the energy region studied are analyzed by fitting the second-derivative spectra d²ε(ω)/dω² to analytic critical-point line shapes. The obtained energies are assigned to certain electronic interband transitions by comparison with existing band structure calculations. © 2000 American Institute of Physics.