The stability of a silica membrane prepared by chemical vapor deposition (CVD) in the HI-H2O gaseous mixture was evaluated aiming at the application for hydrogen iodide decomposition in the thermochemical IS process. Porous alpha- and gamma-alumina tubes having pore sizes of 100 and 10 nm, respectively, were modified by chemical vapor deposition using tetraethoxysilane as the Si source. H-2/N-2 selectivities of the modified membranes which were measured by single-component permeation experiment showed 50.4, 7.5, 63.7, 7.7 and 3.8 at 600degreesC for the NS-1, NS-2, NS-3, NS-4 and S-I membranes, respectively. Stability experiment in the HI-H2O gaseous mixture was carried out at 450degreesC. The prepared membrane using gamma-alumina as the support tube was more stable than that using alpha-alumina as the support tube. The S-1 membrane using alpha-alumina as the support tube showed the high stability in the HI-H2O gaseous mixture and had the high H-2/HI selectivity (240-2600 at 300-600degreesC) in the H-2-H2O-HI (molar composition; 0.09:0.78:0.13) gaseous mixture after the stability test. (C) 2003 Elsevier Science B.V. All rights reserved.