ABSTRACT: Lining of waste storage and disposal sites is a common practice to prevent hazardous waste constituents from migrating out of the facility into the surrounding environment. Recently, geomembranes have been widely used to line the waste storage and disposal sites and are expected to be even more common in the future. However, there are not enough data available on the mass transport of organic chemicals through geomembranes by the physicochemical process of partition and diffusion. In this study, partition and diffusion coefficients were measured using a gravimetric method for high‐density polyethylene (HDPE) geomembrane in acetone, chlorobenzene, ethyl acetate, ethylbenzene, n‐hexane, methylene chloride (MC), n‐octane, toluene, trichloroethylene (TCE), and m‐xylene. Sorption tests were also conducted with mixtures of TCE, MC, toluene, gasoline, and aqueous solutions of TCE, MC, toluene, and m‐xylene. The effect of geomembrane thickness on the mass flux parameters was also evaluated. A mathematical model with a time‐dependent diffusion coefficient successfully simulated sorption curves of pure chemicals. The partition and diffusion coefficients of individual organic compounds in a mixture were accurately predicted from the mass‐transport parameters of individual pure chemicals by multiplying them with their mole fractions. Low‐density polyethylene geomembranes had approximately 4 times and 10 times greater partition and diffusion coefficients than HDPE geomembranes, respectively. A mathematical model together with the experimentally determined parameters was used to predict the mass flux of selected organic chemicals present in gasoline through a 10.2‐cm (4‐in.) HDPE pipe (0.4‐cm [0.16‐in.] thick).