Solubilities were studied for the ternary systems of CsBrLaBr3H2O and CsBrCeBr3H2O at 298.15 K using the isothermal solubility method; the corresponding phase diagrams were constructed. The solid phases were determined by the Schreinemakers wet residues technique. Three solid phases of CsBr, Cs2LaBr5 center dot 10H(2)O, and LaBr3 center dot 7H(2)O in the system CsBrLaBr3H2O, and those of CsBr, Cs2CeBr5 center dot 10H(2)O, and CeBr3 center dot 7H(2)O in the system CsBrCeBr3H2O were obtained, respectively. The compounds Cs2LaBr5 center dot 10H(2)O and Cs2CeBr5 center dot 10H(2)O are incongruently soluble in water. The new solid phase compounds Cs2LaBr5 center dot 10H(2)O and Cs2CeBr5 center dot 10H(2)O were characterized by chemical analysis, X-ray diffraction and thermogravimetric/differential thermogravimetric techniques. The standard molar enthalpies of solution of Cs2LaBr5 center dot 10H(2)O and Cs2CeBr5 center dot 10H(2)O in water were measured to be (49.61 +/- 0.37) kJ center dot mol(-1) and (50.83 +/- 0.46) kJ center dot mol(-1) by microcalorimetry under the condition of infinite dilution, and their standard molar enthalpies of formation were determined as being -(4739.3 +/- 1.3) kJ center dot mol(-1) and -(4729.6 +/- 1.5) kJ center dot mol(-1). The fluorescence excitation and emission spectra of Cs2CeBr5 center dot 10H(2)O were measured. The results show that upconversion spectra of Cs2CeBr5 center dot 10H(2)O exhibit at 524 nm and 697 nm excited at 785 nm, and the intensity of the emission spectra at 524 nm is stronger than that at 697 nm. This research can provide fundamental data for the rare earth industry and future study. The phase diagrams of the ternary systems could provide the fundamental basis and serve as a guide for the preparation of Cs2LaBr5 center dot 10H(2)O and Cs2CeBr5 center dot 10H(2)O.