The phase diagram of the Snl2-CuI system was determined by the method of thermal analysis. No intermediates were found. It is suggested that limiting solid solutions of SnI2 in y-CuI, fl-CuI and ~-CuI, respectively, are formed. The eutectic point parameters (32.7 mol% CuI, 270.44-0.1 ~ and those of two invariant points (64.0 tool% CuI, 385.34-0.5 ~ and 61.5 mol% CuI, 366.64-0.2 ~ corresponding to the equilibria ~+liquid ~ fl and + liquid ~ y, respectively, were determined.Experimental studies on metal iodide systems are rather difficult, primarily because of the thermal stability of iodides compared with that of lower halides. This is probably the reason why only a small number of phase diagrams of copper(I) iodide-metal iodide systems have been published to date. On the other hand, it is found that the copper(I) state becomes more stable as the covalent character of the Cu-X bond increases, e.g. in aqueous halide solutions as halogen is changed from fluorine to iodine [1].The number of published binary phase diagrams of CuI with other iodides does not exceed a dozen. As concerns the alkali metal iodides, the phase diagrams of the LiI-CuI and NaI-CuI systems are not known. Similarly, the HgIz-CuI phase diagram has not been described, though CuzHgI 4 was reported as far back as in the 19th century [2], and has been studied many times during the last forty years with regard to its physicochemical properties.The present phase equilibrium studies on the SnIz-CuI system comprise a continuation of our work on the phase diagrams of the binary systems of SnBr/ with AgBr [3] or with CuBr [4] and ofSnI z with AgI [5,6]. It may be presumed that, together with the chloride systems for these pairs of cations, which were investigated earlier [7][8][9][10][11], a knowledge of the phase diagrams for the complete series of these systems would reveal the more general trends that appear on passing from chlorides to iodides and from copper(I) to silver ions.