The equilibrium solubility of the quaternary system CsCl-LuCl3-HCl-H2O was determined at 298.15 K, and the corresponding equilibrium diagram was constructed. The quaternary system is complicated with five equilibrium solid phases, CsCl, CS4LuCl7 center dot 5H(2)O (4:1 type), Cs9Lu5Cl24 center dot 29H(2)O (9:5 type), Cs2LuCl5 center dot 2H(2)O (2:1 type), and LuCl3 center dot 6H(2)O, of which the new compounds CS4LuCl7 center dot 5H(2)O and Cs9Lu5Cl24 center dot 29H(2)O were found to be congruently soluble and the new compound CS2LuCl5 center dot 2H(2)O was found to be incongruently soluble in the system. The three new compounds obtained were identified and characterized by the methods of X-ray diffraction, thermogravimetry, and differential thermogravimetry. The standard molar enthalpy of solution of CS4LuCl7 center dot 5H(2)O, Cs9Lu5Cl24 center dot 29H(2)O, and CS2LuCl5 center dot 2H(2)O in deionized water was measured to be (15.6 +/- 0.3) kJ center dot mol(-1), (-78.8 +/- 0.6) kJ center dot mol(-1), and (-62.5 +/- 0.5) kJ center dot mol(-1), respectively, by heat conduction microcalorimetry. Their standard molar enthalpies of formation were calculated to be (-4313.0 +/- 2.1) kJ center dot mol(-1), (-17871.6 +/- 5.3) kJ center dot mol(-1), and (-2526.5 +/- 1.2) kJ center dot mol(-1), respectively.