Several ZrO2-Y2O3-SiO2 membranes of which, (ZrO2-Y2O3) content were in the range of 90-100%, were prepared via metal alkoxide method. The sintering temperature was increased with the increase of ZrO2 contents from 1373 for 90% to 2073 K for 100%. On alumina (Tammann temperature is approximately equal to 1500 K) support tubing, two membranes having ZrO2 contents 90 and 92.5% show separation for 50-50% CH4-CO2 gas mixture. On zirconia (Tammann temperature is approximately equal to = 2100 K) support tubings, successful membranes were obtained on the smoothed surface with 40 coating times. Membranes coated with 20 times of ZrO2 = 92.5% solution followed by 20 times of 90% shows separation factor alpha(CH4/CO2) of 1.15 and that with 40 times of 90% 1.08. The composite membranes of ZrO2 content 90% on alumina tubing showed the high permeance for H-2 more than 10(-6) mol/(m(2) Pa s), and peculiarly high permeance for H2O, by 1.7 times more than that expected by Knudsen permeation mechanism. This membrane was tested to separate H-2 from gaseous mixtures of H-2, H2O and HBr, product of thermochemical water decomposition processes, and found that the maximum separation factors for H-2 over H2O and H-2 over HBr were 4.1 and 259, respectively. From the correlation of the pure gas permeances against the reciprocal of square root of the quantity of the absolute temperature times molecular weight, the main permeation mechanism of all gases except H2O are estimated as Knudsen diffusion and main H2O permeation mechanism surface diffusion. It seems that, at higher temperature, permeation mechanism for all gases except H2O shifts to the activated diffusion but for H2O to Knudsen diffusion.