The phase equilibria of the ternary system CsBr + ErBr3 + H2O and the quaternary system CsBr + ErBr3 + HBr (similar to 12.3 %) + H2O at 298.15 K were determined experimentally with the isothermal solubility method. Based on the measured solubility data, the corresponding phase diagrams were plotted. In the ternary system, three crystallization regions corresponding to CsBr, 3CsBr.2ErBr(3).16H(2)O, and ErBr3.9H(2)O were found. Similarly, there were three crystallization regions corresponding to CsBr, 3CsBr.2ErBr(3).16H(2)O, and ErBr3.7H(2)O in the quaternary system. The phase diagrams of the ternary and quaternary systems were compared, and it showed that (1) a new double salt 3CsBr.2ErBr(3).16H(2)O was formed which was incongruently soluble in the two systems; (2) the area of the crystallization region of 3CsBr.2ErBr(3).16H(2)O increased with the increasing concentration of HBr in the equilibrium liquid phase, and (3) ErBr3.9H(2)O transformed into ErBr3.7H(2)O when the HBr reached a certain amount. The new solid-phase compound 3CsBr.2ErBr(3).16H(2)O was characterized by chemical analysis, XRD, and TG-DTG techniques. The standard molar enthalpy of solution of 3CsBr.2ErB(r)3.16H(2)O in water was confirmed to be -(6.69 +/- 0.29) kJ.mol(-1) by microcalorimetry in the limit of infinite dilution and its standard molar enthalpy of formation was determined to be -(7846.1 +/- 1.2) kJ.mol(-1). The fluorescence excitation and emission spectra of 3CsBr.2ErBr(3).16H(2)O were measured. The results indicated that up-conversion spectra of the new solid phase compound exhibit at 470 nm and are excited at 710 nm.