Mutual, solubility and tie-line data are presented for the ternary system, water (W)-phosphoric acid (PA)-1-butanol (BuOH) and water-phosphoric acid-2-butanone (methyl ethyl ketone, MEK) at 25 degrees C, as well as for the quaternary system, water-phosphoric acid-1-butanol-2-butanone, at the same conditions. These systems present an unusual behavior since there are no quaternary tie-line planes and the solubility surface presents a shape like a saddle.