Abstract : Nowadays waste issue is an important stake. The compacted clay and more particular the smectite is the standard material used as pollutant barrier at the bottom of waste landfill. Landfill leachates are characterized by large amounts of ammonium and are composed of several organic molecules. To limit the pollution, the barrier must retain pollutants and limit the permeability. This study is interested in interaction between a montmorillonite exchanged with ammonium cation and short-chain molecule characteristic of those in leachates. The works demonstrated that the ammonium cation in interfoliar position reduces the hydratation ability of the SWy-2(NH4) smectite. The study of the interactions with batch method showed that acetic and formic acids, acetonitrile, dichloromethane, MTBE, benzene, acetone and ethanol are not or very weakly adsorbed on ammonium clay. On the contrary, chloroacetic and oxalic acids, aniline, formamide and phenol are strongly fixed according to several mechanisms. The interactions with carboxylic acids also underlines a weak clay dissolution and a cationic exchange NH4 + to H3O+ / Al3+, notably dependent on the granulometry. The study of the permeability of geochemical barrier by oedometry showed that the ammonium clay and the molecule polluted clay have a higher hydraulic conductivity, which reduces largely barrier efficiency.