The equilibrium solubilities of the quaternary system CsCl-TmCl3-HCl-H2O was determined at 25 ℃ and the corresponding equilibrium diagram was constructed. The diagram is a complicated system with limited solid solutions and formation of a new compound 3CsCl·2TmCl3·14H2O. The new compound has been characterized by XRD, TC-DTG and fluorescence spectrometry. Results show that upconversion spectrum exhibits at 335 nm when exciting at 860 nm, and fluorescence spectrum exhibits at 295 nm and 385 nm when exciting at 240 nm. The compound loses its crystal water by four steps at 50 to 250 ℃. Using a heat conduction calorimeter, the standard molar enthalpy of dissolution of 3CsCl·2TmCl3·14H2O was determined to be (-11.601 ± 0.050) kJ·mol-1 at 298.15 K.
