Na1-xKxMgF3 (0 less than or equal to x less than or equal to 1) solid-solutions were synthesized and the phase diagram of NaMgF3-KMgF3 system was determined by high temperature X-ray powder diffraction experiments and differential thermal analysis (DTA). This system is characterized by a complete series of solid solutions, with a minimum in the solidus at 30 mol% KMgF3 and 1279 K. No immiscibility gap was found. The crystal system changes from orthorhombic to cubic at x = 0.22 at room temperature. The volume change of the unit cell as a function of composition shows a large deviation (excess volume) from Vegard's law for solid solution. The transition temperatures of NaMgF3 and Na0.9K0.1MgF3 from orthorhombic to cubic are 1043 and 723 K, respectively. The transition temperature decreases rapidly by the effect of replacing Na by K. Axial ratios of b/a and clroot12a in orthorhombic NaMgF3 and Na0.9K0.1MgF3 decrease linearly with temperature toward the transition and then discontinuously changes to cubic at the transition point. (C) 2003 Elsevier Science Ltd. All rights reserved.