Using a new robust design model, parametric studies have been performed to investigate the behaviour of gas separation systems based on membrane permeators. The existence of a maximum for permeate purity as a function of membrane pressure ratio is observed for simple permeators without recycle. It is found that overpurification of the permeate stream, followed by mixing (a by-pass configuration) may be profitable in cases where membrane area is ex-pensive compared to compression, and purity requirements are modest. Retentate recycle is found to be detrimental to product purity and recovery in both retentate and permeate streams. It is shown that permeate recycle may significantly reduce compressor duty, even in cases where recycle is not required to obtain the specified permeate purity.