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One of the functions of capping systems is to control the emission of gases into the atmosphere. Not only these gases are toxic for human health and the environment, but some of them like methane can be valued as a source of energy. However, most geomembrane types, such as LLDPE, offer only limited barrier performance against methane permeation. In addition, the characterization of gas permeation through geomembranes is currently being performed using 40 year old measurement methods mostly based on manometric or volumetric techniques. This has led to research and development efforts to improve the measurement of the gas permeability of geomembranes, which is a critical tool to allow the development of geomembranes with enhanced barrier properties against methane and carbon dioxide. This paper reviews the advantages and weaknesses of the methods currently available for measuring the permeability of geomembranes to gases. It also presents a new method proposed for measuring the permeability of geomembranes to methane and carbon dioxide. This method involves the control of the concentration of methane or carbon dioxide on one side of the geomembrane, and the measurement of the amount of gas having diffused through the material to the other side of the geomembrane. This technique permits the determination of gas permeation through geomembranes at various concentrations of gases and temperatures, while allowing a control of the total pressure on both sides of the geomembrane.