The water-phosphoric acid-cyclohexane ternary system was studied at 25 and 35 degrees C under atmospheric pressure, and the binodal curve was determined by the cloud-point method. In addition, the isoplethic section between 2H(3)PO(4)center dot H2O-cyclohexane was investigated using a Tammann diagram based on the thermal analysis of different samples with the determination of hemihydrate melting temperature. The concentration of phosphoric acid and cyclohexane was assayed by volumetric titration and gas chromatography, respectively; the amount of water was calculated by the material balance equation. The miscibility between phosphoric acid and cyclohexane was found to be very low with the appearance of a three-phase stability domain liquid-liquid-solid by the presence of phosphoric acid crystals: hemihydrate (2H(3)PO(4)center dot H2O) and anhydrous phosphoric acid (H3PO4).