Solid-liquid phase equilibria have been studied at 273 K for the chloropinnoite + boric acid + water system. The equilibria solid phases identified by infrared spectra and X-ray diffraction were inderite (2MgO center dot 3B(2)O(3)center dot 15H(2)O) when the mass percentage of H3BO3 was in the range from (0 to 1.5) %, hungtsaite (MgO center dot 2B(2)O(3)-9H(2)O) from (1.5 to 5.5) %, and macallisterite (MgO center dot 3B(2)O(3)-7.5H(2)O) when the mass percentage of H3BO3 was higher than 5.5 %. The pH values of the equilibria liquid phases and the solubility data of the corresponding solid phases were determined, respectively. The mechanisms of the phase transformation of chloropinnoite in, boric acid aqueous solution are proposed and discussed.