In the system ZnS/ZnSe/CdS/CdSe mixed crystals have been prepared in presence of iodine vapor as a mineralisator gas. Crystallographic parameters of the mixed crystals have been determined by X-ray powder methods. Mixed crystals with high Zn content crystallize in the sphalerite-structure, other ones in the wurtzite-structure. Molar volumes derived from experimentally determined lattice constants can be given in form of a plane as a function of the composition of the mixed phases. Vegard's law is often discussed in binary systems as a straight line. The corresponding plane for the system ZnS/ZnSe/CdS/CdSe is presented here. (C) 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved.