The effect of layer thickness on the structural and electro-optical properties of MOVPE grown lattice matched Zn1-yMgySxSe1-x/GaAs layers were examined by high resolution X-ray diffractometry (HRXRD) and cathodoluminescence (CL) combined with secondary electron (SE) imaging. Our X-ray results show that the lattice parameter and the strain status depend on the layer thickness, although the quaternary layers were grown with nominally the same growth parameter. All samples have a high dislocation density observed by panchromatic CL image (p>10(5) cm(-2)). Furthermore, inhomogeneous emission intensity on CL micrographs and wavy surfaces on SE images were also observed for all samples. With increasing layer thickness the contrast of intensity inhomogeneities in CL becomes stronger, while a significant lattice parameter variation is observed.