The influences of the acidity/basicity on the phase composition and the crystallite size of zirconia powders prepared in the NaNO3/KNO3 system was investigated by adding Lux-Flood bases (NaNO2/KNO2, Na2O2 and Na2CO3) to the system. It was found that the reaction occurred at lower temperatures in the system containing bases than in the pure NaNO3/KNO3 system. The phase composition of the tetragonal zirconia yielded varies with the concentration of the bases contained in the NaNO3/KNO3 melt. When the base concentration is above a certain value, 100% tetragonal phase can be produced. The crystallite size of the tetragonal phase decreases with increasing base concentration, i.e. basicity, in the melt. The crystallite size, was found to decrease in the order of added bases in the molten NaNO3/KNO3 system : Na2O2 > Na2CO3 > NaNO2/KNO2. The strength of the basicity of the commonly used molten nitrate and nitrite systems was found to decrease in the order : NaNO2/KNO2 > LiNO3/KNO3 > NaNO3/KNO3 and NaNO3. It appears that the acidity/basicity of a melt plays a very important role in the phase composition and crystallite size during the precipitation of zirconia from molten salts.