The metastable zone width of boric acid in aqueous solutions was measured in the presence of Ca+2 Mg-2, Cr+3, Fe+2 ions. For the measurements of metastable zone width, the polythermal method was used. The impurity concentration is: varied from 0 to 1000 ppm in each aqueous solution. It was: found that the range of metastable zone width of boric acid in aqueous solutions is considerably affected by impurities. The width of metastable zone of aqueous solution of boric acid is: reduced with increasing impurity concentration. In the presence of Ca+2 and Cr+3. the equilibrium saturation concentration is changing.