Optical properties of CdyZn1-yS nanoparticles differing by their size and composition are presented. The fluorescence quantum yield increases with increasing particle size and composition (y). The photoluminescence, PL, spectra are due to defect states and follow the quantum size effect. A red-shift in the fluorescence spectrum with increasing cadmium composition and particle size is observed which is attributed to a transition from the valence band to a deeply trapped hole. Both absorption and fluorescence spectra in various media and at temperatures from 77 to 300 K are presented.