(Solid + liquid) equilibria in the ternary system CsCl + HoCl3 + H2O and in the quaternary system CsCl + HoCl3 + HCl (similar to 10%) + H2O at T = 298.15 K and atmospheric pressure were investigated by using isothermal solution saturation and moist residue method. Based on the measured solubility data, the corresponding phase diagrams were plotted. In the ternary system, three crystallization regions corresponding to CsCl, Cs2HoCl5 center dot 6H(2)O and HoCl3 center dot 6H(2)O were found. However, there were four crystallization regions corresponding to CsCl, Cs5HoCl8 center dot 4H(2)O, Cs2HoCl5 center dot 6H(2)O and HoCl3 center dot 6H(2)O in the quaternary system. The phase diagrams of the ternary and quaternary systems were compared, and it showed that (1) new solid-phase compound Cs2HoCl5 center dot 6H(2)O was obtained which was congruently soluble in the two systems; (2) a new double salt Cs5HoCl8 center dot 4H(2)O was formed in the quaternary system which was congruently soluble in an average medium of similar to 10 mass% HCl, and (3) the area of the crystallization region of CsCl decreased with the increasing concentration of HCl in the equilibrium liquid phase. The new solid-phase compounds Cs5HoCl8 center dot 4H(2)O and Cs2HoCl5 center dot 6H(2)O were characterized by chemical analysis, XRD and TG-DTG techniques. The standard molar enthalpies of solution of Cs5HoCl8 center dot 4H(2)O and Cs2HoCl5 center dot 6H(2)O in water were confirmed to be (53.09 +/- 0.53) kJ . mol(-1) and (4.26 +/- 0.24) kJ . mol(-1) by microcalorimetry in the condition of infinite dilution and their standard molar enthalpies of formation were determined to be -(4530.1 +/- 1.4) kJ . mol(-1) and -(3776.6 +/- 0.9) kJ . mol(-1). The fluorescence excitation and emission spectra of the new solid phase compounds were measured. The results indicate that upconversion spectra of the compounds Cs5HoCl8 center dot 4H(2)O and Cs2HoCl5 center dot 6H(2)O at the same wavelength 474 nm, when the former was excited at 701 nm and the latter was excited at 713 nm. (C) 2016 Elsevier Ltd. All rights reserved.