Superionic MSnF4 are the highest performance fluoride ion conductors, with PbSnF4 being the best. Prototypes of devices using PbSnF4 have been constructed and tested. Since the fluoride ion mobility is thermally activated, some devices might be used more efficiently above ambient temperature. Therefore, it is of prime importance that the stability of these materials be tested under potential conditions of use, since thermal degradation and phase transitions are likely to alter the conducting properties. Tetragonal SrSnF4, α-PbSnF4 and BaSnF4, and orthorhombic o-PbSnF4, are stable at ambient conditions, even in air. However, all undergo significant deterioration when heated in air: the color changes from white to yellowish, and tin hydrolysis and oxidation takes place. They are much more stable under inert conditions (nitrogen or argon). However, PbSnF4 undergoes several phase transitions at high temperatures: o to α starting at ca. 100°C, α to β (reversible, but the reverse reaction is very sluggish) starting at 250°C, β to γ (reversible) at 390°C. β-PbSnF4 can be quenched to ambient temperature and is metastable for long times, however, eventually it starts changing to stable α-PbSnF4 and this change is uncontrollable and is faster above ambient temperature. Microcrystalline μγ-PbSnF4, obtained by ball milling any other phase of PbSnF4, gives rapidly a-PbSnF4 at 200°C. In addition, for all MSnF4, hydrolysis of the Sn-F bonds to Sn-O occurs with traces of moisture.