Spontaneous nucleation of potassium chloride crystals onto the surfaces of dissolving sodium chloride substrate crystals was observed in the ternary system of NaCl-KCl-H2O. The phenomena were understood being controlled by the local conditions of the mother liquor adjacent to the dissolving surface of the sodium chloride substrate and are specific to ternary systems or systems having more than two components, since the local supersaturations are created by the equilibrium conditions. When sodium chloride crystals are dissolving in either aqueous potassium chloride solutions or NaCl-KCl-H2O solutions, the solution adjacent to the dissolving surface is assumed to be saturated with respect to sodium chloride, and this can cause local high supersaturation with respect to potassium chloride enough to induce its nucleation. The mechanism of nucleation of potassium chloride is discussed on the basis of ternary phase equilibrium and kinetics of dissolution. (C) 2002 Published by Elsevier Science B.V.