A new class of HZ selective membrane, based on CsHSO4, was produced using an extremely simple preparation procedure comprising an impregnation-, a drying- and an uniaxial pressing-step. The optimized preparation conditions resulted in membranes showing hydrogen permeance up to 0.2 mu mol s(-1) m(-2) Pa-1 and a permselectivity of 14 and 7 towards H-2:CO2 and H-2:CH4, respectively. All gas permeation experiments were performed between 20 and 180 degrees C. TGA, XRD and impedance spectroscopy were used to identify the various phases of CsHSO4. Interesting is that the hydrogen permeance of the membrane is related to the specific phases of CsHSO4. The gas transport mechanism in CsHSO4-SiO2 composite membranes seems to be a combination of Knudsen diffusion and solution diffusion through the monoclinic phases of CsHSO4. The proton conductive properties of CsHSO4 play an insignificant role in the hydrogen transport through the membrane. (c) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.