Solubility was studied in the CsCl + YbCl3 + HCl + H2O quaternary system along the (similar to 11.3 %) HCl section at 298.2 K using the isothermal solubility method, and the corresponding phase diagram was plotted. The compositions of solid phases were determined by the Schreinemaker's wet residues technique. The system was complicated with four equilibrium solid phases CsCl, Cs4YbCl7 center dot 4H(2)O, Cs2YbCl5 center dot 5H(2)O, and YbCl3 center dot 6H(2)O. The compound Cs4YbCl7 center dot 4H(2)O was incongruently soluble, and the compound Cs2YbCl5 center dot 5H(2)O was congruently soluble in the system. The new solid phase compounds were characterized by chemical analysis, X-ray diffraction (XRD), and thermogravimetric/differential thermogravimetric (TG-DTG) techniques. The standard molar enthalpies of solution of Cs4YbCl7 center dot 4H(2)O and Cs2YbCl5 center dot 5H(2)O in water were measured to be (18.19 +/- 0.47) and (13.22 +/- 0.27) kJ . mol(-1) by microcalorimetry under the condition of infinite dilution, and their standard molar enthalpies of formation were determined as being - (4039.2 +/- 2.2) kJ . mol(-1) and - (3469.2 +/- 1.3) kJ mol(-1), respectively.