Liquid-liquid equilibrium for the ternary system water/phosphoric acid/diisopropyl ether was studied at the temperatures (273.15, 283.15, and 293.15) K. The concentration of each phase was determined by acidimetric titration, the Karl Fisher technique, and nuclear magnetic resonance. Phase diagrams with tie lines and plait points are reported; the latter are calculated by Hand's method. The behavior of the three-phase zone was also studied; results show that the size of this zone decreased as the temperature decreased.