Phase studies were undertaken with PbFCl-type rare earth oxychlorides (REOCls), where RE = La-Dy and Y. Specifically, equilibria in REOCl-H2O systems were investigated in an effort to describe the interrelationship of the oxychloride, hydroxychloride, and trihydroxide phases of rare earths in water. When lighter rare earths were employed, REOCIs were generally stable in basic media, while acidic solutions readily yielded RE(OH)(3) by hydrolysis. However, the systematics of equilibrium established in sufficient amounts of water did not continue across the rare earth series and were not particularly relevant to cases in which the oxychloride phase was added to a volume of water smaller than the critical quantity. Difficulties encountered during initial attempts to investigate the hydrolysis of REOCl required that a special parameter, the dilution ratio (D = volume of water/weight of powder), be used for REOCl-RE2(OH)(5)Cl center dot nH(2)O RE(OH)(3) phase relationships in water at different temperatures. The equilibration of REOCl in solutions of low dilution ratios (D <= 0.5 mL/mg) yielded all three phases depending on the nature of the RE, dilution ratio, and temperature. Interestingly, the addition of salt (NaCl) to systems at similar dilution ratios markedly increased the stability of RE2(OH)(5)Cl center dot nH(2)O phases. In contrast, the essential feature of YOCl-water equilibria in the range 0.1 <= D <= 1.0 mL/mg was characterized by spatial stability of the Y-2(OH)(5)Cl center dot nH(2)O structure. In conclusion, the present investigation provides systematic phase diagrams for rare earth oxychloride water systems that can be used as guidelines for the preparation and application of REOCIs and RE2(OH)(5)Cl center dot nH(2)O phases in aqueous media.