Mineral liners may interact with landfill leachate, and this interaction may lead to integrity loss and structural changes that may cause an increase in permeability and a decrease in adsorption capacity. In unlined landfill sites, the leachate may interact with clays used as geological barriers. In this case, micro structural changes in the underlying clays may cause settlement and bearing capacity problems, which should be taken into account if the landfill site is reused. The research presented in this paper aims to demonstrate the changes in micro structure, permeability, compressibility, and stress-strain behaviour of leachate-exposed Istanbul clays. Undisturbed and reconstituted samples of three different clays were subjected to different combinations of water and landfill leachate as permeants. Changes in the clay structure were interpreted by detailed mineralogical, chemical, and scanning electron microscopy image analyses, which were coupled with index, swelling, consolidation, and strength properties. Because the clays had different contents of illite/smectite mixed layer minerals, initial structure, and bonding types, they were affected by the leachate at different levels. It was shown that the macro behaviour was affected by mineral disintegrations and transformations, which led to destructuralisation, particle re-alignment and subsequent structure gaining. (C) 2011 Elsevier B.V. All rights reserved.