High temperature electrochemical synthesis is a very promising method for the production of refractory boride powders and coatings. The Na2B4O7-Na3AlF6 binary system is part of the more complex NaCl-Na3AlF6-Na2B4O7-TiO2 electrolyte proposed for TiB2 electrosynthesis. A new approach by high temperature and high resolution solid state nuclear magnetic resonance was employed for the structural description of this system, from the solid at room temperature to the liquid at 1100 degreesC. The change in Al-27, Na-23, B-11, and F-19 NMR spectra with composition give evidence for the insertion of aluminum into the glassy network of Na2B4O7 as (AlO4] and diluted [AlF6] units as well as formation of [BOF2] groups into solidified mixtures. A devitrification mechanism of glassy Na2B4O7 under influence of Na3AlF6 addition and temperature was discussed in terms of lowering boron coordination and obtaining oxyfluoride species.