The objective of this study is to identify the effects of changing the permeation rate, reaction rate and selectivity on the exit conversion of a membrane reactor, and to provide a guideline in designing membrane in terms of selectivity and permeability. Two dimensionless groups, Pe and Da, are employed to represent the relative magnitude of permeation rate and reaction rate to convection, respectively. For both isothermal and non-isothermal operation, the contour of the exit conversion is plotted as a function of Pe and Da. Four distinctive regions are identified where the conversions are controlled by different mechanisms. Selective permeation is the controlling mechanism for mid-range Pe. Increasing the selectivity of the membrane increases the selective permeation controlled regime. Plot of the exit conversion as a function of Pe and selectivity is developed and used to test the validity of increasing selectivity at the expense of permeability. Increasing the sweeping gas flow rate increases the exit conversion. However, the conversion has a value under unity with finite selectivity in the selective permeation regime. Results from the non-isothermal operation of a membrane reactor shows a maximum exit conversion with Da since the reverse reaction becomes marked at the end of the reactor as Da increases. But this effect is diminished, as the sweeping flowrate becomes large.