Equilibrium phase diagrams for the systems NdCl3–CaCl2and NdCl3–NaCl were determined by differential thermal analysis. A simple eutectic was observed at 59 ± 1 mol% CaCl2and 600°± 2°C in the NdCl3–CaCl2system. A compound NaCl.3NdCl3which melts incongruently at 545°± 5°C to NdCl3and a liquid containing approximately 47 mol% NaCl, and a eutectic at 68 mol% NaCl and 439°± 2°C were found in the NdCl3–NaCl system. On the basis of agreements between the activities calculated by the Clausius–Clapeyron equation and Temkin's model using the present data for the NdCl3–CaCl2system and the literature data for the PrCl3–CaCl2system, the melts in the former system consist of Nd3+, Ca2+, and Cl−ions and in the latter system of Pr3+, Ca2+, and Cl−ions. The above approach indicates the presence of Na+, Cl−, and NdCl2‐5ions in the NaCl‐rich melts and Nd3+, Cl−, and NdCl−4in the NdCl3‐rich melts in the NdCl3–NaCl system. Analogous ions were indica