stabilised by chemical substitution so as to occur within the standard operating range of a multi-anvil press, is briefly discussed. For NaMgF 3 , the transitions to the high-pressure phases occur at pressures outside the normal range of a multi-anvil press. Two different sequences of transitions had previously been suggested from computer simulations. With increasing pressure, we find that the relative stability of the NaMgF 3 phases follows the sequence: perovskite → CaIrO 3 structure → Sb 2 S 3 structure → P6 3 /mmc structure. However, only the perovskite and CaIrO 3 structures are stable with respect to dissociation into NaF and MgF 2 .