To control membrane plasticization in natural gas streams containing higher hydrocarbon compounds, we propose to coat a hydrocarbon-based polymer gas separation membrane with a fluoropolymer layer to reduce the hydrocarbon penetrant partial pressure to which the hydrocarbon membrane is exposed. A theoretical analysis is presented which addresses the trade-offs between reducing membrane plasticization, reduction in gas flux through the composite membrane and reduction in product purity due to the fluoropolymer layer. The analysis reveals that an effective plasticization-resistant, hydrocarbon-fluorocarbon composite membrane would consist of a fluoropolymer that has a lower ratio of hydrocarbon gas to CO2 solubility and a higher size-selectivity than the hydrocarbon polymer. Model cases are presented to illustrate the possibilities and limitations of this approach.