The gas separation characteristics of ultramicroporous glass capillary membranes were investigated. Glass capillary membranes which had ultramicropores (pore diameter less than 1 nm) were prepared by elution of alkali metal ions from glass capillaries, It was found from elution treatment that alkali metal ions of 3.5 Na (composition : 77.8 SiO2, 22.2 Na2O mol%) and 3.5 K (composition : 77.8 SiO2, 22.2 K2O mol%) glass capillaries were completely eluted after 90 and 10 minutes with 3 mol/L HNO3, 240 and 10 minutes with 3 mol/L CH3COOH and 3 mol/L H3PO4, respectively. Permeations of N-2, He, and CO2 were measured at 298, 373, and 473 K. Permeation rates of the 3.5 Na and 3.5 K glass capillary membranes were 1-10(-1) (cm(3)(STP).cm(-2).s(-1).cmHg(-1)). The ratios of permeation rates, CO2/N-2 and He/N-2, were similar to the theoretical Knudsen values.