This paper considers the design of multistage membrane plants operating in the diffusion controlled region where gas flux is proportional to the differences in the gas partial pressure across the membrane. The flow inside the modular stages was assumed to be fully mixed and the stages were connected in a simple countercurrent mode. The design method was thus particularly aimed at separations involving inorganic membranes. A flexible algorithm was used to define the interstage flow sequence and the plant was assumed to be built of modules of the same size; only the number of modules assigned to a stage was allowed to vary. Trial and error was used to obtain convergence of the plant mass balance. The criteria for optimisation are discussed. Predictions of module behaviour for the fully mixed and the more realistic plug flow cases are considered in an appendix. Based on this comparison, the applicability of the current design methods for stage cuts above 0.2 is questionable.