A method to produce relatively high concentrations of intermediate components from isotope mixtures in a separation cascade with local extension of a working substance flow at its inner stages is demonstrated. The cascade under consideration consists of two square parts of different width and has 4 external flows: the feed and waste ones and the end and interjacent product flows. The research demonstrates that the cascade with flow extension built by simple reshaping of a square cascade enables provide considerable increasing an intermediate component concentration in an interjacent product flow in comparison with the conventional 4 flows square cascade having the same values of the total and additional flows. In the example, it was examined enrichment of the W-183 isotope for which an increase in concentration provided by a cascade with flow extension is about 5 abs% whereas the compared cascades keep at their end product flows almost the same concentration of separating components. (c) 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.