The gas permeability of H2S gas at 150 degrees C through ultra-thin cesium hydrogen sulfate (CsHSO4) membranes has been investigated. Gas chromatography-mass spectrometry analyses indicate that CsHSO4 membrane is impermeable to H2S gas under test conditions. The apparent micropore diameter of the membrane averaged between 9.5 and 11.5 angstrom with a maximum permeance of 0.09 Barrer (6.75 x 10(-19) m(2) s(-1) Pa-1). Atomic force microscope and X-ray diffraction analyses show respectively that the surface morphology and crystal structure of the membranes are preserved, with no adverse effect from prolonged exposure to H2S gas. Electrochemical impedance spectroscopy analysis confirm over a 30% decrease in membrane resistance via an 80% reduction in membrane thickness. (c) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.