The effect of penetrant diffusion on the barrier properties of PVC geomembranes have been determined for the following organic environmental contaminants : benzene, dichloromethane, and trichloroethylene. The membranes experienced swelling to a degree which depends on the type of penetrant used. Some dissolution may occur, and in general we are dealing with Case II transport. The experimental work was performed with an ASTM-cell, a Cahn balance, and an in-house built gravity (G-) cell. Those instruments generated comparable data on breakthrough times. The obtained diffusion coefficients as well as the breakthrough times obeyed an Arrhenius-type relation over the temperature range studied. Liquid sorption of the various penetrants modified the geomembrane structure. Membrane surface pretreatment with different contaminants influences the subsequent transport of organic penetrants through PVC geomembranes. That is to say : the induced swelling, as result of membrane contact with one penetrant, is likely to alter the system free volume, allowing for a different rate of mass transport for subsequent penetrants.