A continuous, mixed suspension, mixed product removal, vacuum crystallizer was built and operated to study the effect of addition of NaCl and MgCl(,2), separately and combined, on the crystallization kinetics of KCl. An analysis technique proposed by previous investigators and based on the concept of the population balance was employed to obtain the growth and nucleation rates at various operating conditions for KCl.;The tendency of KCl to form polycrystals was confirmed. It was found that the presence of sodium chloride or magnesium chloride in small concentrations markedly reduced the tendency toward polycrystals. It was also found that the presence of both magnesium and sodium ions had a much greater effect on the reduction of polycrystal formation. Moreover, the presence of magnesium ions reduced the amount of sodium chloride on the potassium chloride crystalline product.;The formation of potassium chloride polycrystals and the reduction of this polycrystalline formation in the presence of NaCl and MgCl(,2) was explained in terms of a sub-nuclei model. The effect is related to the solution structure forming properties of magnesium and sodium ions. This explanation was supported by the reduced presence of sodium in the potassium chloride crystals when magnesium ions are present in the crystallizing solution and on the observation by other investigators who have studied the crystallization of other materials.