The electronic and optical properties of the II-VI semiconductors ZnS, ZnSe, US, CdSe and their alloys Zn1-xCdxSySe1-y have been are investigated using the empirical pseudopotential method. For the quaternary alloy ZnCdSSe, we have coupled the virtual crystal approximation with the pseudopotential method. The energies of the Gamma, X, and L conduction valleys of the Zn1-xCdxSySe1-y alloys as a function of y were calculated. The electronic structures of Zn0.9Cd0.1S0.2Se0.8 and Zn0.9Cd0.1S0.15Se0.85 alloys were studied for lattice matching to GaAs as well as ZnSe. Also the composition dependence of the refractive index and the dielectric function of II-VI quaternary alloys were also calculated. (C) 2003 Elsevier B.V. All rights reserved.