Abstract: When a n-type CdS single crystal is treated with Se vapor at 600-700℃, a layer of CdS_xSe_1-x (0<x<1) forms on the surface. The composition and the properties of photolu-minescence (PL) and electroluminescence (EL) of the samples depend on the Se vapor pre-sure, temperature and the time of the reaction. A red shift of emission bands were observed when X decreases. When the surface layer is thin enough, the EL spectra show the bands come from very near the surface (the diffusion layer) only and the PL spectra show the bands come both from the diffusion layer and the substrate. The fwhm of the emission bands of inhomogeneous samples (> 30 nm) is remarkably broader than that of homogeneous samples (single crystals). The Auger electron spectra connected with Ar ion sputter etching show that the thickness of the diffusion layer is about 0.2-2 μm under our experimental conditions.