The solubilities of saturated solns. of quaternary systems: MCl-GdCl3-HCl-H2O (M = K, Rb, Cs; 20癈) at different acid concns. have been determine and the corresponding equilibrium diagrams were constructed. When M = K, at the parallel sections of HCl%(wt) = 14 - 23, no compounds were formed in the system; when M = Rb, at HCl%(wt) = 13, no new phases were discovered, but at the parallel section of HCl%(wt) = 22 the incongruently soluble compound RbCl譍dCl3?H2O (I) was formed; when M = Cs, at HCl%(wt) = 13 and 20 of saturated solns., two typical compounds 4CsCl譍dCl3譎2O (II) and 2CsCl譍dCl3?H2O (III) were formed. The compound II is incongruently soluble compound at the two acid concns. With increasing acid concentration, the transition of compound III from congruently soluble to incongruently soluble occurs. The compounds I, II and III have been also characterized by X-ray diffraction and thermoanal. The compounds I and II have not yet been reported in the literature.

Key words: PHASE EQUILIBRIUM, WATER, HYDROCHLORIC ACID, QUATERNARY SYSTEM, POTASSIUM CHLORIDE, CESIUM CHLORIDE, RUBIDIUM CHLORIDE, GADOLINIUM CHLORIDE