The solubility and ultra solubility of boric acid (H₃BO₃) in solutions with different concentrations of MgCl₂ and NaCl were determined by using laser detection technique．In addition, the metastable zones of H₃BO₃ in these solutions were obtained. Furthermore, the nuclear progression m of H₃BO₃ was calculated. Our experimental results showed that the stable area of H₃BO₃ was narrow at high temperature, and wide at low temperature. Compared to the solubility of H₃BO₃ only in water, the solubility of H₃BO₃ decreased with increasing amount of MgCl₂ and NaCl in water and was more affected by MgCl₂ than by NaCl. The metastable zone of H₃BO₃ first became narrower and then wider with increasing concentrations of MgCl₂ and NaCl．The metastable zone of H₃BO₃ in MgCl₂ and NaCl solutions was narrower than in the H₃BO₃-H₂O system. The effect of MgCl₂ on apparent nucleus progression of H₃BO₃ was much more than that of NaCl. The crystal grew better in medium concentration of MgCl₂ and low concentration of NaCl because of the smaller nuclear progression．Therefore, crystal with greater granularity can be obtained in such buffer condition．

Key words: MgCl₂, NaCl, H₃BO₃, metastable zone, laser