Series of the mixed fluorides with the general composition M 1 - x Nd x F 2 + x (0.00=<x=<1.00; M=Sr 2 + and Ca 2 + ) were prepared by a vacuum heat treatment of the appropriate mixtures of MF 2 (M=Ca and Sr) and NdF 3 . The products obtained were analyzed by powder X-ray diffraction, which revealed the phase relations in these systems. At the MF 2 -rich compositions a fluorite-type solid solution was observed in both systems. The typical solid solution limits of NdF 3 in the SrF 2 and CaF 2 lattice are about 40 and 45mol%, respectively. The incorporation of NdF 3 in the fluorite lattice of CaF 2 causes an expansion of the unit cell volume. The unit cell volume of the fluorite lattice in Sr 1 - x Nd x F 2 + x system surprisingly remains constant throughout the solid solution range. The NdF 3 (tysonite) type phase separates out beyond the solid solution limit, in both systems. Also, it was observed that about 15 and <10mol% of CaF 2 and SrF 2 , respectively, could be retained in the NdF 3 , maintaining the tysonite lattice. No fluorite or tysonite-related ordered phases were observed in these systems.