Membrane reactor performance in reversible reactions like dehydrogenation is often compared to plug-flow reactor performance. Analysis of cocurrent membrane-reactor design equations shows that as the Damkohler-Peclet product varies from 0 to infinity, the membrane reactor approaches a fully diluted or undiluted plug-flow reactor. If its yield is maximized by varying this parameter, the optimum cocurrent membrane reactor is often one of these limiting cases, and a membrane reactor offers no advantage over a plug-flow reactor. When the optimum membrane reactor does not correspond to one of these limiting cases, there still may be a plug-flow reactor system that offers equal or higher yields. Analysis of plug-flow-reactor performance indicates that there is an optimum degree of dilution; while the optimized membrane reactor yield may be greater than either an undiluted or a fully diluted plug-flow reactor; it may not be greater than an optimally diluted plug-flow reactor. When using porous membranes the membrane reactor yield is at most 7% greater than the yield using a plug-flow reactor. The porous membranes enhance the yields in a regime where the reactor would not be likely to operate. Much larger yield advantages are possible, but only when the membrane is highly permselective, as with dense membranes. Highly permselective membranes also enhance the yields in a regime where the membrane reactor process is likely to be operated.