Iodine excess separation from hydriodic acid (HI) is one of the most challenging steps of the Sulfur-Iodine thermochemical water splitting cycle. One promising method is the extraction of HI by using phosphoric acid (H3PO4), with the subsequent separation of gaseous hydriodic acid from water and H3PO4 by a distillation step. The aim of the present work is to provide new experimental liquid-liquid equilibrium data for the biphasic HI/I-2/H2O/H3PO4 quaternary system, varying both temperature and solution composition in order to optimize the excess of anhydrous phosphoric acid to be added. Two temperature levels were tested, i.e. 100 degrees C and 120 degrees C, and the H3PO4 amount was varied in the feed mixture from 7.7% wt to 38% wt while the [I-2]/[HI] and [H2O]/[HI] molar ratios were kept constant at, respectively, a value of 3.7 and 5.6. A temperature level of 120 degrees C, with an H3PO4 initial concentration of about 29% wt leads to the lowest amount of water against HI, which minimizes energy costs in the phosphoric acid reconcentration step, the most energy consuming part in this separation process. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.