Solubility and liquid-liquid phase equilibrium data are presented for the ternary system water-phosphoric acid-methyl isobutyl ketone at 25 and 40 degrees C. Binodal curves, tie lines and distribution curves have been determined. Hand’s method has been used to correlate tie lines and to calculate coordinates of plait points. The heterogeneous region at 40 degrees C appeared to be slightly broader than that at 25 degrees C, and the slopes of tie lines somewhat higher : the solubility of this system increases with decreasing temperature. The distribution curves indicated that methyl isobutyl ketone was an effective extractant for concentrated phosphoric acid solutions only.