We discuss the optical analysis of Zn(Se,S) layers by the application of far-infrared (FIR) and Raman spectroscopy. First a summary is presented of the principles of FIR and Raman Spectroscopy with regard to the relevant epilayer properties which can be investigated by these methods. Subsequently we treat some selected results of FIR and Raman analysis of epilayers grown on GaAs(100) by metal organic vapour phase epitaxy (MOVPE). The main points of interest are crystalline quality, free carrier concentration and interface sharpness. It is shown that n-type conductivty is achieved by Ga implantation in ZnSe but the annealing process (870°C, 30 s) leads to a p-type interface layer in the GaAs due to Zn diffusion. Furthermore, the beneficial effect of growth interruptions to the sharpness of ZnS/ZnSe multiquantum-well interfaces is demonstrated.