Synthetic geomembranes are commonly used as barriers or covers in hazardous waste management facilities. A confined double-compartment method (CDCM) was developed to evaluate the sorption and diffusion mechanisms of volatile organic compounds (VOCs) through high-density polyethylene (HDPE), very low-density polyethylene (VLDPE), and polyvinyl chloride (PVC) geomembranes. The loss of VOCs during testing was measured and its effect on mass transport parameters assessed. The capacity of geomembranes to adsorb VOCs, indicated by weight gain, was large, ranging from 21% to 52% of the initial geomembrane weight. In general, the total amount of VOCs lost decreased as the aqueous concentration decreased and increased as the test duration increased. The VOC loss in the CDCM test was within the maximum gas Chromatograph analytical error of about 3% to 4%; thus, the CDCM enables sorption onto and diffusion through a geomembrane to be evaluated accurately along with the influence of concentrations above and below the geomembrane on the transport behavior.