Phase transitions in the miscible solid solution Na1-xKxMgF3 were examined over a wide range of compositions by computer molecular dynamics, X-ray diffraction, and thermal analysis in order to characterize the polymorphic phase transitions as a function of alkali-metal content. The pure NaMgF3 composition has a single orthorhombic-to-cubic transition at 1038 K, but computer modeled compositions with K+ partially substituted for Na+ ions have at least two polymorphic transitions. The models also indicate that the transition temperatures decrease with increasing potassium content, Results from thermal analyses and from literature give similar results, Computer simulations, experimental data, and literature values all show a room-temperature transition for the composition around Na0.65K0.35MgF3.