center dot The solubility data of the quaternary system CsCl-TbCl3-HCl(similar to 7.6 mass %)-H2O at 298.2 K were determined, and the corresponding phase diagram according to central projection data on the trigonal basal face CsCl-TbCl3-H2O was plotted. The diagram includes three invariant points and four crystallization fields corresponding to CsCl, Cs5TbCl8 center dot 6H(2)O, Cs2TbCl5 center dot 6H(2)O, and TbCl3 center dot 6H(2)O. Two new solid-phase compounds of Cs5TbCl8 center dot 6H(2)O and Cs2TbCl5 center dot 6H(2)O, which are congruently soluble in the system, were isolated and characterized by the methods of powder X-ray diffraction, thermogravimetric, and differential thermal analysis. The standard molar enthalpies of solution for the two compounds of Cs5TbCl8 center dot 6H(2)O and Cs2TbCl5 center dot 6H(2)O were measured to be (60.64 +/- 0.28) and (14.08 +/- 0.45) kJ center dot mol(-1) by heat conduction microcalorimetry under the conditions of infinite dilution, and their standard molar enthalpies of formation were calculated to be -(5087.1 +/- 1.7) kJ center dot mol(-1) and -(3764.2 +/- 1.0) kJ center dot mol(-1). The fluorescence excitation and emission spectra of Cs5TbCl8 center dot 6H(2)O and Cs2TbCl5 center dot 6H(2)O were measured. The results show that upconversion spectra exhibit for Cs5TbCl8 center dot 6H(2)O at 500 nm and 545 nm and for Cs2TbCl5 center dot 6H(2)O at 495 nm and 545 nm excited at 750 nm, and the upconversion intensity increases with the increase of TbCl3 contents in CsCl.