Separation of the gas mixtures 7.7 vol % nitrogen in ammonia and 5.5% carbon monoxide in ammonia in one-compressor multistage membrane devices with one and two separation steps has been compared. The apparent values of selectivity have been determined for these gas mixtures. Expressions relating the degree of separation to the stage cut and the degree of purification to the product loss have been experimentally obtained. The degree of purification has been shown to increase with the number of stages despite of the fact that the degree of separation at individual stages is lower than in a single membrane module.