The equilibrium solubility of the quaternary system LuCl3-CdCl2-HCl-H2O was determined at 298.15 K, and the corresponding equilibrium diagram was constructed in this paper. The quaternary system is complicated with four equilibrium solid phases, CdCl2 center dot H2O, Cd3LuCl9 center dot 14H(2)O (3:1 type), CdLuCl5 center dot 9H(2)O (1:1 type), and LuCl3 center dot 6H(2)O, of which the new compounds Cd3LuCl9 center dot 14H(2)O and CdLuCl5 center dot 9H(2)O were found to be congruently soluble in the system. The two new compounds obtained were identified and characterized by the methods of X-ray diffraction, thermogravimetry, and differential thermogravimetry. The standard molar enthalpies of solution of Cd3LuCl9 center dot 14H(2)O and CdLuCl5 center dot 9H(2)O in deionized water were measured to be 28.79 +/- 0.22 kJ.mol(-1) and -15.48 +/- 0.30 kJ.mol(-1), respectively, by heat conduction microcalorimetry. Their standard molar enthalpies of formation were calculated to be -6427.5 +/- 1.3 kJ.mol(-1) and -4133.7 +/- 0.7 kJ.mol(-1), respectively.