Abstract: A method of and apparatus for separating a lighter gas from a heavier gas in a gas mixture. The method involves directing generally annular jets of a jet gas into the gas mixture, and extracting the lighter gas from the gas mixture through central zones of the generally annular jets. The apparatus comprises a jet membrane separating a chamber for the gas mixture and a chamber for the gas enriched in the lighter gas, the jet membrane utilizing annular jet orifices communicating with the gas-mixture chamber and utilizing extraction tubes at the jet orifices and interconnecting the two chambers. A jet gas delivered to the jet orifices is discharged therethrough into the gas-mixture chamber, and the lighter gas of the mixture passes through the extraction tubes into the chamber for the gas enriched in the lighter gas. By cascading a number of jet membrane stages of the foregoing nature, a very high degree of purity for the lighter gas may be achieved.