A sol-gel-derived microporous ceramic membrane with an exceptionally low permeability for CO(2) from gaseous streams was developed and characterized. The sols were prepared from a mixture of niobium and silicon alkoxide precursors by acid-catalyzed synthesis. Microporous films were formed by coating asymmetric gamma-alumina disks with the polymeric sol (Si/Nb=3:1), followed by calcination at 500 degrees C. The membrane consists of a 150-nm-thick layer with a Si/Nb atomic ratio of about 1.5. The single-gas permeance of small gas molecules such as H(2), CH(4), N(2), and SF(6) decreases steadily with kinetic diameter. Hydrogen, helium, and carbon dioxide follow an activated transport mechanism through the membrane. The permeance of CO(2) in this membrane is much lower than that in pure silica, and its behavior deviates strongly from the general trend observed with the other gases. This is attributed to a relatively strong interaction between CO(2) and adsorption sites in the niobia-silica membrane.