Transmittance measurements of layered mixed crystals were performed in the 1.80-2.80 eV photon energy range at . Band-gap energies of the studied crystals were estimated by means of the derivative spectra of transmittance and photon energy dependence of absorption coefficient. The compositional dependence of direct band-gap energy at revealed that as sulphur (selenium) composition is increased (decreased) in the mixed crystals, the direct band-gap energy increases from 2.19 eV () to 2.67 eV (). Photoluminescence spectra of mixed crystals were investigated in the photon energy region of 2.00-3.10 eV at . The observed bands were assigned to the transitions of electrons from conduction band to the shallow acceptor levels in the band gap. The shift of the PL bands to higher energies with increasing sulphur content was revealed. In addition, the composition ratio of the mixed crystals was obtained from the energy-dispersive spectroscopy experiments.