Oligo-Miocene clay outcrops on the European side (west and northwest part) of Istanbul were analysed. Formerly, a landfill and sanitary landfill were built on the clay. Mineral liners of the current and extending parts of the Istanbul landfill consist of these clays, since they include a considerable amount of smectite, illite, and kaolinite. With this feature, these clays are also an important candidate for the buffer material of repositories for nuclear wastes of newly planned nuclear power plants. In this context, one Miocene and two Oligocene clay samples were subjected to leachate under low stress using an odometer device during a period of 30 days, 180 days, and 360 days to understand the chemical and mineralogical transformations and subsequent changes in the clay structure. The results of this work and our ongoing other research revealed that Istanbul clays are mostly illite/smectite mixed-layer minerals. Illites considerably increased while the illite/smectite mixed-layer minerals decreased in the first 15-30 days. The kinetics of the three clays was studied to understand the reasons for the illite increase. Increase of the activation energy over time may be attributed to the successive intercalation of illite lattice layers as alteration of mixed-layer illite-smectite clays. Mineral dissolution, however, is still the primary mechanism for illitization when the low activation energy is considered. With these findings, the utilization of Istanbul clays is questionable for clay barriers of landfills or sealing material of hazardous wastes.