A system of isolated localized exciton states corresponding to particular energies from the tail of the density of states is observed in CdS _{1 - x}Se_x crystals. From an analysis of the obtained photoluminescence excitation spectra, the boundary between the free and localized excitons and the length of the tail region of the density of localized states that make an essential contribution to radiative recombination are determined. It is shown that the localized and free excitons prove to be separated and represent two exciton subsystems. The energies of actual optical photons are accurately determined to be 23.6 ± 0.2 and 35.6 ± 0.3 meV for CdS_0.70Se_0.30 and 20.7 ± 0.2 and 31.4 ± 0.3 meV for CdS_0.50Se_0.50.