The system diopside-nepheline-leucite, representing a join in the undersaturated part of the system nepheline (~e)-kalsilite (Ks)-CaO-MgO-SiO 2, has been investigated at atmospheric pressure. The system is pseudoternary and cuts the primary phase volumes of forsterite solid solution (Foss), diopside solid solution (Diss), ncpheline solid solution (Ness), camcgieite solid solution (Cgss), and leueite solid solution (Less). Melilite (Mel) occurs as a subliquidus phase. The phase diagram has two four-phase points: 1. one at 1275 •176 and Di60NesLcs2 where liquid coexists with Foss, Diss and Less, corresponding to olivine (O1) lcucitite; 2. the other at 1194~=5~ and Di2,.sNe29.sLc43 where Ness, Foss and Less coexist with liquid, corresponding to Ol-~e italite. With decreasing temperature, liquid moves from point (1) to a five-phase assemblage (3) where liquid is in equilibrium with FOss, Diss, Mel and Less (1258~5~ which is representative of O1-Mel-leucititc. From point (2) liquid moves to a second five-phase assemblage (4), where FOss, Mel, Ness, Less and liquid are in equilibrium (1175=J=5~ corresponding to a Lc-Ne katungite. The assemblage FOss+ ~ess-{-Diss-F Mel + Less-I-liquid, is reached between 1168 ~ and 1100~ and corresponds to OI-Mel-Ne leucitite. FOss reacts with liquid and disappears. Near the point (1) it disappears at 1135 g-10~ whereas near the point (2) it reacts out at 1060~ 10~ Near the join Di-Ne it disappears at 950 ~= 10~ The final assemblage in the system is representative of Mel-Ne leucitite. * Presented at the symposium "Recent Advances in the Studies of Rocks and Minerals at High Pressures and Temperatures" held in Montreal, 1972. Jointly sponsored by the International Mineralogical Association and the Commission on Experimental Petrologic. lo*