Solid oxide membrane (SOM) electrolysis process can produce high-purity silicon from SiO2 dissolved in molten oxy-fluoride flux at elevated temperatures. Yttria-stabilized zirconia (YSZ), the preferred material for the oxygen-conducting membrane for this application, is found to degrade over time upon exposure to the silica-containing molten oxy-fluoride flux. This YSZ degradation is caused by the acidity of the dissolved silica, especially when the optical basicity of the molten flux is lower than that of the yttria present in the YSZ membrane. To counteract this mismatch, the addition of CaO, a basic oxide, to the flux can adjust the optical basicity of the flux and successfully mitigate the YSZ membrane degradation. The detailed correlation between the rate of YSZ membrane degradation and the optical basicity of the flux is investigated by systematically testing a series of flux compositions. It is found that as the oxide optical basicity in the flux approaches that of the yttria in the YSZ, the degradation of the YSZ membrane is mitigated and essentially vanishes when the flux acidity with respected to the yttria is neutralized. This approach provides a guideline for eliminating membrane degradation during the production of silicon using the SOM electrolysis process.