Hydrogen separation in H-2-H2O-HI gaseous mixture using the silica membrane prepared by CVD was evaluated aiming at the application for hydrogen iodide decomposition in the thermochemical IS process. Porous alumina tubes having pore size of 100 nm (M1) and 10 nm (M2) were modified by chemical vapor deposition using tetraethoxysilane as the Si source. Single-component permeance to He, H-2, and N-2 was measured at 300-600 degrees C. Hydrogen permeance of the modified membranes at a permeation temperature of 600 degrees C was about 6 x 10(-9) mol/Pa m(2) s. H-2/N-2 selectivities at 600 degrees C were 5.2 and 160 for M1 and M2 membrane, respectively. Separation experiments of a H-2-H2O-HI mixture using the modified membranes were carried out at 300-600 degrees C. Hydrogen permeances were almost the same as the single-component permeances, whereas HI permeances were below 1 x 10(-11) mol/Pa m(2) s. The hydrogen permeances were not changed after one day exposure in a mixture of H-2-H2O-HI with a molar composition of 0.23:0.65:0.12 at 450 degrees C.