CdS_1-xSe_x crystals of two different compositions (x = 0.5 and 0.25) have been investigated by high resolution transmission electron microscopy and cathodoluminescence measurements. The first time, it could be established that inside the single CdS_1-xSe_x crystals (hexagonal structure), many stacking faults exist which correlate to lamellae of cubic structure. Independent of their chemical composition intrinsic stacking faults of types I₁ and I₂ are the predominant defects in all the crystals examined. Moreover, a noticeable difference in the cathodoluminescence spectra could be observed for regions mainly having a hexagonal structure and from regions containing stacking faults.