In this paper, the spectroscopic ellipsometry measurements on TlGaS2xSe2(1 - x) mixed crystals (0 <= x <= 1) were carried out on the layer-plane (001) surfaces with light polarization E perpendicular to c* in the 1.2-6.2 eV spectral range at room temperature. The real and imaginary parts of the dielectric function, refractive index and extinction coefficient were calculated from ellipsometric data using the ambient-substrate optical model. The critical point energies in the above-band gap energy range have been obtained from the second derivative spectra of the dielectric function. Particularly for TlGaSe2 crystals, the determined critical point energies were assigned tentatively to interband transitions using the available electronic energy band structure. The effect of the isomorphic anion substitution (sulfur for selenium) on critical point energies in TlGaS2xSe2(1 - x) mixed crystals was established. (C) 2012 Elsevier B.V. All rights reserved.