The MF-AlF3-Al2O3 (M = Li, Na, K) systems have been studied in the liquid state. Raman spectra were recorded to obtain information on the structure and the ionic composition of these melts, vapor pressure measurements were performed to obtain thermodynamic information, and the Al2O3 solubility has been measured as a function of the melt composition and temperature. These data show that the ions AlF4-, AlF52-, AlF63-, and F- exist together with the oxyanions Al2OF62-, Al2O2F42-, and possibly Al3O4F43-. For the NaF- and KF-containing systems, the solubility of Al2O3 increases very much with the mole ratio n degrees(MF)/n degrees(AlF3) and is maximum at n degrees(MF)/n degrees(AlF3) = 4 (8.1 mol % at 1040 degrees C for the NaF-AlF3 mixtures, 16.8 mol % at 1027 degrees C for the KF-AlF3 mixtures). For the LiF-AlF3 system the Al2O3 Solubility is much smaller with a maximum of approximate to 2.2 mol % probably around n degrees(MF)/n degrees(AlF3) = 1.5 at 1000 degrees C. The four equilibria AlF63- = AlF52- + F-, AlF52- = AlF4- + F-, 2Al(2)OF(6)(2-) + 2F(-) = Al2O2F42- + 2AlF(5)(2-), 2Al(2)O(2)F(4)(2-) + F- = Al3O4F43- + AlF52-, can give a reasonable explanation of the observed Raman, vapor pressures, and solubility data.