MFI‐type zeolite (ZSM5) membranes were prepared on alumina and stainless steel tubular supports and tested for the separation of CO₂/N₂ mixtures. The effect of several operating variables (temperature, feed and permeate pressures, feed composition, use of sweep gas, presence of water in the feed, membrane module configuration) was investigated. The separation between CO₂ and N₂ takes place because of the preferential adsorption of CO₂, which hinders the permeation of N₂ through the zeolite pore network. The best results in this work were obtained with a Na‐ZSM5 membrane, and using a large pressure gradient. In this case, a permeation flux as large as 9.2 kg/(m² · h) [permeance of 2.6 × 10⁻⁶ mol/(m² · s · Pa)] with a CO₂/N₂ separation factor of 13.7 was obtained. © 2004 American Institute of Chemical Engineers AIChE J, 50: 127–135, 2004