Solid-liquid phase equilibria of the 2MgO center dot 2B(2)O(3)center dot MgCl2 center dot 14H(2)O + 3.0 % H3BO3 + H2O system have been studied at various temperatures of (0 to 200) degrees C. The solid phase equilibria identified by infrared spectra and X-ray powder diffraction were MgO center dot 2B(2)O(3)center dot 9H(2)O at (0 to 27) degrees C, 2MgO center dot 3B(2)O(3)center dot 15H(2)O (kurnakovite) at (27 to 50) degrees C, 2MgO center dot 3B(2)O(3)center dot 15H(2)O (inderite) at (50 to 72) degrees C, MgO center dot B2O3 center dot 3H(2)O at (72 to 98.5) degrees C, 2MgO center dot B2O3 center dot 2H(2)O at (98.5 to 190) degrees C, and 2MgO center dot B2O3 center dot 1.5H(2)O at (190 to 200) degrees C, respectively. The pH values of the liquid phase equilibria and the solubility data of the corresponding solid phases were determined. The formation mechanisms of the solid phases were proposed and discussed.