The feed and permeate pressures achievable in most gas separations are limited by economics. As a result, commercial membrane separation processes usually operate with a feed-to-permeate pressure ratio (theta) in the range of 5-15. This limited pressure ratio means that the optimum membrane may not be the one with the highest selectivity; rather, a membrane with lower selectivity, but tailored to the pressure ratio, may result in a more economical process. In other words, when a process is pressure ratio limited, the useful selectivity is also limited. The balance between pressure ratio and selectivity is often ignored by membrane developers. The importance of this balance is illustrated with two applications of current interest: CO2 capture from power plant flue gas, and dehydration of azeotropic bioethanol. (C) 2014 Elsevier B.V. All rights reserved.