This paper describes the concept of "targeting" the development of new membranes such that the resulting membrane separator will be economically viable in a particular commercial process. The separation of carbon monoxide and hydrogen for the production of acetic acid is used to illustrate the development of a new membrane material using the targeting procedure. It is shown by economic evaluation of the complete process that the cost of separation can be related to the permeability and selectivity of the membrane. Costs upstream and downstream of the separator are taken into account. The same cost may be achieved by many different combinations of membrane permeability and selectivity; therefore, iso-cost lines can be drawn on a plot of permeability and selectivity which provides a guide for the development of new membranes. A new membrane material has been selected for the separation of carbon monoxide and hydrogen for the acetic acid process. The copolymer was synthesized and dense homogeneous membranes were manufactured for the measurement of gas transport and physical properties. The membrane has substantially different gas transport properties from currently used commercial membranes. If this membrane was developed, it would be expected to result in a reduction in the cost of carbon monoxide separation to 40% of the cost of the currently used method, cryogenic distillation.