In the present paper

we report excitation spectra of the free exciton emission on high purity ZnS

x

Se

1-x

single crystals at x=0.02 and 0.125.Results indicate that the exciton migration is reduced by the excjton-localiza-tion induced by the atomic scale compositional-fluctuation of S and Se atoms within the exciton radius.Highly pure ZnS

x

Se

1-x

single crystals are grown by the sublimation method.Sample surfaces are carefully etched and cooled to 2 K by immersing it into liquid helium.Grating-monochromatized light beam from a 2 kW Xe-1amp(FWHM-1 meV)is utilized for the excitation source.The resulting excition luminescence lines are analyzed by a J.Y.U-1000 double mo-nochromator.