In this study, the (solid + liquid) phase equilibria for the two systems of CsBr + EuBr3 + H(2)Q and CsBr + EuBr3 + HBr (similar to 12.3%) + H2O at 298.15 K and atmospheric pressure were determined by isothermal solubility method. Schreinemaker wet residue approach was used to determin the solid phases obtained from the studied systems. The related phase diagrams were,depicted based upon the experimental data. There were two invariant, points, three invariant curves, and three crystallization fields corresponding to CsBr, 5CsBr center dot 2EuBr(3)center dot 20H(2)O, and EuBr3 center dot 6H(2)O in the two phase diagrams. New double salt 5CsBr center dot 2EuBr(3)center dot 20H(2)O was incongruently soluble in the two systems. The content of EuBr3 in two invariant points decreased as. the concentration of HBr in equilibrium liquid phase increased, while that of CsBr changed a little. The new solid-phase compound 5CsBr center dot 2EuBr(3)center dot 20H(2)O was investigated using chemical, XRD, and TG-DTG analyses. Additionally, the standard molar enthalpy of solution for SCsBr center dot 2EuBr(3)center dot 20H(2)O in water was confirmed to be 104.69 +/- 0.88 kJ/mol by microcalorimetry; its standard molar enthalpy of formation was determined as being -(9659.7 +/- 1.6) kJ/mol. The fluorescence excitation and emission spectra of 5CsBr center dot 2EuBr(3)center dot 20H(2)O were acquired. The results indicate that upconversion spectra of SCsBr center dot 2EuBr(3)center dot 20H(2)O exhibited pure green upconversion luminescence under 785 nm excitation.