Mass spectrometric determination of absolute pressures of the positive Na + , K + , Cs + and the negative NaAl 2 F 8 − , AlF 4 − , Al 2 F 7 − , NaAl 3 F 11 − , Na 2 Al 3 F 12 − ions in the saturated vapor over the Na 5 Al 3 F 14 –AlF 3 system was carried out at the temperature near 850K. The measurements were made in a clean nickel effusion cell and in the same cell with the inner surface covered by cerium trifluoride. An increase of negative ion pressures at least three order of magnitude has been observed after CeF 3 treatment as compared with a clean cell. At that time the pressure of positive ions decreased slightly. The result is explained by F − superionic conductivity of CeF 3 .