The aim of this paper is to find out the optimum configuration and design variables for the asymmetric membrane-based separation of carbon dioxide from natural gas for meeting the pipeline specification of 2% carbon dioxide. For this purpose, optimization of gas processing cost of the membrane unit having up to three stages is performed based on a fundamental model of the unit. It shows that there is no unique configuration that is always optimum irrespective of the values of carbon dioxide concentration and natural gas price. However, within certain ranges of the carbon dioxide concentration and the natural gas price, the optimum configuration may be unique and the minimum gas processing cost can be achieved by adjusting only the number of modules in each stage and the compressor power. In most cases, there is no significant cost difference between the two and three staged optimum configurations and the choice of a configuration may depend on engineering assessment. (c) 2006 Elsevier B.V. All rights reserved.