In France, geosynthetic clay liners (GCLs) can be used under certain conditions within liner systems for waste containment. But there still remains a lot to learn regarding the potential modifications undergone by GCLs during prolonged contact with landfill leachate. In 2001, the French Environmental Agency (ADEME) initiated the LIXAR research program aimed at examining, at different scales of observation and for different experimental conditions, the modifications undergone by GCLs during prolonged contact with leachate. This paper focuses on the permeability-diffusion tests performed as part of this research program. Two types of needle-punched GCLs, a natural sodium bentonite GCL and a sodium-activated calcium bentonite GCL, were put in contact with three types of fluids in oedopermeameter cells. Contact duration was between 4 and 6 months. Results provide information regarding the relative performance of the two GCLs and the influence of the pre-hydration and percolation fluids.