The distribution of isomorphous salts between aqueous and solid phases infractional crystallization was determined. The results were examined from the standpoint of theoretical equations developed by Doerner and Hoskins, Berthelot and Nernst, and Abu Elamayem. Calculations of the distribution constants, X and D, employed in the Doerner and Hoskins and Berthelot and Nernst equations, respectively, show that these equations are not strictly applicable to the systems studied. Experimental results can be quantitatively explained, however, by Abu Elamayem's equation. The less soluble salt is separated more effectively by allowing a small weight fraction of the crystals to separate from a ternary system.