A detailed optical characterization of zinc-blende-type Zn 0.93Mg0.07Se and Zn0.81Be0.04Mg 0.15Se mixed-crystal samples grown by using the modified high pressure Bridgman method has been carried out via photoluminescence (PL), contactless electroreflectance (CER) and photoreflectance (PR) in the temperature range of 15 to 400 K. PL spectra at low temperatures consist of an exciton line, a broader emission band due to recombination of donor-acceptor pairs and a broad band related to recombination through deep-level defects. Interband transitions, originating from the band edge and spin-orbit splitting critical points, have been observed in the CER/PR spectra. The optical properties of these two mixed crystals are compared and the results indicate the superior quality of the Be-incorporated mixed quaternary crystals. The peak positions of the excitonic emission lines in the PL spectra correspond quite well to the energies of the fundamental transitions obtained by means of electromodulation data. The parameters that describe the temperature dependence of the fundamental and spin split-off band gaps and the broadening function of the band-edge excitons are evaluated and discussed.