The Bunsen reaction for the production of hydriodic and sulfuric acids from water, iodine and sulfur dioxide, has been studied with the evaluation of the effect of some operative parameters on product phase behavior. Results show that operative temperature has a minor effect on the phase behavior. In contrast, both iodine and water loads can be adjusted to enhance the downstream operations of the sulfur-iodine thermochemical water-splitting cycle: the effect of iodine and water excess on resulting phases purity, side reaction occurrence and acid concentration was studied and, then, the most favorable operative conditions defined. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.