The separation power of ultrapurification of gases in cascades of the continuous membrane column type was calculated. The equation for the degree of separation of the extraction and enrichment section was obtained. The total area of membranes in continuous membrane column cascades is compared with that of nonmixing multistage countercurrent cascades. The maximum degree of separation of a mixture of argon and propane as an impurity attained in the apparatus was 8.5 x 10(4), with 98% product extraction.