The effect of fiber property variations on the performance of a two-stage system for gas separation was studied theoretically and experimentally. Variations in fiber inner diameter, slow gas permeance, or selectivity are considered. Variability in any of these properties is detrimental to performance, as characterized by product flow rate and product recovery (fraction of the feed recovered as a product), The performance decline increases as either property variation or product purity increase. The two-stage system can enhance performance significantly over a single sage. Permeate miring is also beneficial to performance. Like a single stage, however, some fibers may stop producing a retentate product and consume product produced by other fibers for sufficiently large variation in size and permeance. The theory agrees well with experiments for the production of nitrogen from air.