In the present work, the purification of the wet process phosphoric acid (WPA) with mixtures of methyl isobutyl ketone (MIBK) and tri-butyl phosphate (TBP) was investigated. Based on a three step purification process, the optimal solvent mixture composition that yielded the highest purified acid with the greatest possible P2O5 recovery was obtained for an MIBK percentage of 55%. Acid recovery increased for higher solvent MIBK contents whereas the impurities' contents were lower for near equal TBP and MIBK percentages in the binary solvent mixture. The phase diagram of the ternary system H3PO4-water-optimal solvent mixture was determined. The partition ratios for H3PO4 as well as three trace metallic impurities were also obtained. Relative purification was better for Mg. The lowest selectivity was that of Fe which, unlike for the other trace elements does not improve when the solvent rate increases. The best purifications are obtained for the higher solvent mixture rates. While TBP has the highest selectivity, compared with MIBK, the solvent mixture is more selective for extracting H3PO4. (c) 2007 Elsevier B.V. All rights reserved.