The concentration of hydriodic and sulfuric acid aqueous solutions by membrane distillation (MD) was experimentally investigated. Two commercial hydrophobic membranes, with two different recirculation batch configurations, were tested: direct contact membrane distillation (DCMD) with a polypropylene (PP) capillary membrane, and air-gap membrane distillation (AGMD) with polytetrafluoroethylene (PTFE) flat-sheet membrane. Feed temperatures were 58 degrees C for DCMD and 80 degrees C for AGMD, with cooling water at 15 degrees C and 1 atm operating pressure in both cases. H2SO4 concentration in the feed solution increased from 1.1 up to 7.0 mol/L with the DCMD and from 0.9 up to 10.1 mol/L with the AGMD. HI concentration increased from 0.3 up to 7.0 mol/L with the DCMD and from 0.3 up to 8.0 mol/L with the AGMD. The latter value is higher than the azeotropic concentration of the HI/H2O (7.57 mol/L) mixture and, hence, HI further separation from water can be easily achieved with conventional distillation units. Durability of PTFE membrane in acid solutions was assessed too. (c) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.