AbstractCalorimetric measurements have been performed on mixed crystals of couprous and mercury iodide at a wide range of compositions. From these investigations specific heats, transition enthalpies and finally the phase diagram CuI — HgI2were derived. The separating lines of different phase regions were calculated from enthalpies of transition versus composition diagrams. In this way the homogeneous region of the existing compound Cu2HgI4could be determined too.