The solubility diagrams of the system Na[La‐EDTA]–Na[Pr‐EDTA]–H2O (EDTA = ethylendiamine‐tetraacetate) have been established at 0°, 35 ° and 50°. A miscibility gap occurs a t 0° between the solid phases Na[(La, Pr)‐EDTA] 6 H2O and Na[(Pr, La)‐EDTA], 8H2O, and also at 50° between the latter and Na[(La, Pr)‐EDTA], 3H2O. At 35° however, a continuous series of mixed crystals Na[(La, Pr)‐EDTA], 8H2O appears as the solid phase in this system.