[en] The effect of an axially varying countercurrent circulation rate in a gas centrifuge on the efficiency factors, e/sub I/ (the ideality efficiency) and e/sub C/ (the circulation efficiency), is investigated. The optimum value of the centrifuge parameter, m, which is a measure of the countercurrent circulation rate, is determined as a function of axial position in the centrifuge. Corresponding values of the centrifuge separation efficiency are presented. Numerical examples based on the centrifuge model recommended for use by the Workshop Committee are presented