This paper presents the new concept of ’Targeting’ the properties of new membrane materials. Cost parameters are defined in terms of effective selectivity and cost permeability. These new parameters, obtained from the analysis of membrane plant suppliers’ bids, are used in a procedure which determines the optimal degree of separation and recovery. The use of the targeting methods is illustrated here in the development of new membranes for the separation of CO from an H-2/CO syngas for the large-scale manufacture of acetic acid. The targeting approach can be extended to include other gas separation technologies and will aid the development of a generic methodology for the design of gas separation processes.