Abstract: The invention provides a method of operating a cascade gas separation process and a gas separation cascade (10) useful for uranium refining and employing a carrier gas. The method and cascade of the invention involve arranging relative gas flow rates between at least some modules (12) in the cascade so that the relative proportions of process gas leaving and entering each of said modules are such that there is a substantially constant molar flow rate of gas in each direction along the cascade through said modules. The proportion of carrier gas relative to process gas increased in opposite directions from the feed (14) to the opposite ends (20, 22) of the cascade and the modules through which the constant molar flow rate takes place have substantially the same size, capacity and construction.