Boehmite AlO(OH) sol was prepared by sol-gel method from aluminum iso-propoxide under an acidic condition and this sol was coated on an alumina support. After it was dried for 2 days at room temperature and calcined at

for one hour. The membrane had homogeneous ultra fine pores, about 3.0nm, with a narrow pore size distribution and the thickness was in the range of

, depending upon the number of coatings. Gas permeation characteristics of the membrane with

thickness were investigated for four gases He,

. Knudsen diffusion was the dominant transport mechanism for these gases. But

showed much higher permeabilities than the values predicted by Knudsen diffusion due to the effect of surface diffusion. The adsorption energy of

obtained from adsorption iso-therms was 22.40kJ/mol and the calculated surface diffusion coefficients of

turned out to be

and

at 290K, respectively. The additional flows by surface diffusion of

was about 43% and 22% of the total flows, respectively. The separation factors of

mixtures approached the theoretical values predicted by Knudsen diffusion with increasing temperature.