The solubility in the RbCl-TmCl3-HCl-H2O and RbCl-LuCl3-HCl-H2O quaternary water-salt systems was studied along the similar to 22 % hydrochloric acid section by the isothermal solubility method at 298.15 K, and corresponding equilibrium diagrams were constructed. The compositions of solid phases were established by the Schkreinemakers' wet residue method. The quaternary system RbCl-TmCl3-HCl-H2O is a simple eutonic type, but the quaternary system RbCl-LuCl3-HCl-H2O is complicated, and one incongruently saturating complex phase is formed in this system: Rb2LuCl5 center dot 7H(2)O. The compound Rb2LuCl5 center dot 7H(2)O was identified and characterized by the methods of powder X-ray diffraction, thermogravimetric, and differential thermal analysis. The crystal water was lost at (308 to 499) K. The standard molar enthalpy of solution of Rb2LuCl5 center dot 7H(2)O in water was measured to be (6.04 +/- 0.17) kJ.mol(-1) by heat conduction microcalorimetry, and its standard molar enthalpy of formation was calculated to be -(4009.9 +/- 1.3) kJ.mol(-1).