Single crystals of A3MF6 (A = Rb, Cs; M = Al, Ga) were grown from mixed alkali chloride/fluoride fluxes in sealed silver tubes. For Cs3AlF6 and Cs3GaF6, two polymorphs were observed at room temperature: m-Cs3MF6 and o-Cs3MF6. For the two Rb containing compositions, only one room temperature polymorph was observed: o-Rb3AlF6 and t-Rb3GaF6, respectively. Simultaneous TGA/DSC and high temperature SCXRD/PXRD were used to study the high temperature behavior of A3MF6. The compounds of all four compositions were found to undergo structure transitions upon heating to the same cubic structure type, c-A3MF6.