All natural gas must be dried before it enters distribution pipelines, to control corrosion and prevent formation of solid hydrocarbon/water hydrates. Glycol dehydrators are often used for natural gas dehydration, which, however, causes the emissions of hazardous volatile organic compounds (VOCs). In light of increasing concerns about VOC emissions, this study examines the technical feasibility of membrane technology for natural gas dehydration. More specifically, the Part I of this study is focused on the laboratory-scale tests of composite membranes. First, the current state-of-the-art research in membrane materials for H2O/CH4 separation is reviewed. Hydrophilic microphase-separated block copolymers (Pebax (R)) with promising H2O/CH4 separation properties were identified and made into industrial thin film composite membranes. Second, the composite membranes were extensively tested in a laboratory permeation cell by varying operating parameters including feed gas composition, feed gas flow rate and the flow rate of the dry sweep gas on the permeate. Water vapor permeation was mainly restricted by the microporous support and paper layers of the composite membranes, as opposed to methane permeation, in which the main transport resistance was in the selective layer. Increasing sweep gas flow rate on the permeate side of the membrane decreased the overall membrane transport resistance to water vapor, thus increasing water vapor permeance (up to 2000 gpu) and H2O/CH4 selectivity (up to 1500). Finally, the effect of different layers of composite membranes on water vapor permeance and water/methane selectivity is interpreted using a resistance-in-series model. The Part II of this study will describe a field test of Pebax (R)-based membranes at a natural gas processing plant and membrane process designs to maximize the membrane separation efficiency. (C) 2012 Elsevier B.V. All rights reserved.