Solubility of the quaternary system RbCl-SmCl(3)-HCl(12.84, 22.66 % by mass)-H(2)O was determined at 298.15 K, and corresponding equilibrium diagrams were constructed. The quaternary system is complicated with three equilibrium solid phases RbCl, RbSmCl(4)center dot 4H(2)O (1:1 type), and SmCl(3)center dot 6H(2)O. The solubilities of RbCl, RbSmCl(4)center dot 4H(2)O, and SmCl(3)center dot 6H(2)O decreased with an increase of the concentration of HCl. The crystallization field decreased for RbCl and SmCl(3)center dot 6H(2)O and increased for RbSmCl(4)center dot 4H(2)O with an increase in the HCl concentration. The new solid-phase compound RbSmCl(4)center dot 4H(2)O was found to be uncongruently soluble in the system. The compound RbSmC(4)center dot 4H(2)O was characterized by the methods of X-ray diffraction, thermogravimetry, and differential thermogravimetry. The compound loses its crystal water by one step at (359 to 457) K. The standard molar enthalpy of solution of RbSmCl(4)center dot 4H(2)O in deionized water was measured to be -(29.40 +/- 0.26) kJ.mol(-1) by heat conduction microcalorimetry. Its standard molar enthalpy of formation was calculated to be -(2725.3 +/- 1.0) kJ.mol(-1).