||Conventional clay liner, which is widely used as a hydraulic barrier to water, has been shown to be adversely affected by organic fluids. However, the addition of quaternary amines into bentonite greatly enhances its compatibility with organic fluids and thus allows the clay barrier technology to be extended to the treatment of organic contaminants. In this study, an organically modified clay, organoclay BB-40 (dicetyldimethylammonium treated bentonite), was studied for using it as a secondary containment for petroleum underground storage tanks, in order to prevent the surrounding soil and groundwater from being contaminated by leakage. Both the small-scale and large-scale permeability tests reveal that the hydraulic conductivity of organoclay BB-40 to gasoline is on the order of 10-9 cm/set under a hydraulic gradient around 90. This implies that it will be an effective hydraulic barrier to prevent the advective migration of gasoline. In addition, organoclay BB-40 has a good resistance to freeze-thaw and dry-wet cycles due to its large swelling capacity in petroleum products, indicating that it has a good self-healing ability to cure liner defects. Moreover, batch sorption tests reveal that organoclay BB-40 has high sorption capacities for benzene, toluene, ethylbenzene, and xylene (BTEX). The large-scale tests also verify the substantial uptake of hydrocarbons by organoclay BB-40 that organics as high as 150% of the dry weight of organoclay are retained in the liner. It denotes that organoclay BB-40 can act as a chemical barrier against the movement of organic contaminants. All the results suggest that organoclay can be a successful barrier for petroleum storage tanks to prevent soil and groundwater contamination.