The fluorite type MF2 materials react at high temperature with tin(II) fluoride SnF2 in the 1: 1 stoichiometry to give high performance fluoride ion conductors MSnF4. However, the reaction takes place only for the largest M cations (M = Sr, Pb and Ba). For the others (M = Cd and Ca), no reaction takes place. Furthermore, when the reaction occurs, it is observed that the reactivity of SnF2 with MF2 is highly dependent on M. The reaction with PbF2 is the easiest; it is complete at 250°C, whereas 500°C is required with SrF2 and BaF2. In addition, with SrF2, an incomplete reaction is observed very often. Moreover, PbF2 and SnF2 fully react at other stoichiometries to give other materials, whereas SrF2 and BaF2 do not, and with CdF2 and CaF2 not even MSnF4 is formed. A structural analysis based on bond angles and using the size of the M2+ cations is used in an attempt to explain the lack of reactivity for the smallest M2+ cations and the poor reactivity of SrF2. However, the relative cationic sizes cannot explain all the trends, inclusing the exceptionally high reactivity of PbF2 (lower reaction temperature, much richer source of materials). An explanation based on the electronic structure of tin(II) and lead(II) has been developped in order to account for the results.