Gas/gas membrane contactors are devices in which a feed gas and a sweep gas are circulated on either side of a membrane. The pressures of the two gas streams are often approximately equivalent, so the principal driving force for permeation is the concentration difference between the feed and sweep gas components. This type of contactor has been commercialized for energy recovery devices in air conditioning applications. More recently, these contactors have been suggested for use in carbon dioxide capture and sequestration. In this paper, the performance of an ideal contactor using perfectly selective membranes is examined. For this type of ideal contactor, analytical equations can be derived that allow the partial pressure profiles within the contactor to be calculated. The effect of the sweep ratio (sweep Row rate/feed flow rate) and feed pressure on separation performance of the contactors has been calculated. In the final section of the paper, the performance of contactors fitted with membranes permeable to other components of the feed gas is described. (C) 2014 Elsevier B.V. All rights reserved.