The effect of the nature of the bentonite (natural sodium or calcium activated) and of cation exchange on diffusive properties of two geosynthetic clay liners are examined. Diffusion laboratory tests were performed to estimate the diffusion coefficients of 1,2-dichloroethane (DCA), dichloromethane (DCM), thrichloroethylene (TCE), toluene and benzene. Both GCLs are needle-punched and come from the same manufacturer. The nature of the bentonite contained in both GCLs was different as a natural sodium bentonite and a calcium activated sodium bentonite were tested. Furthermore, one of the GCLs experienced cation exchange through contact with a synthetic leachate in order to generate an increase in its hydraulic conductivity by a factor 8.5. The objective of this process was to evaluate if an increase in hydraulic conductivity by cation exchange in a GCL would result in a significant increase in diffusion coefficients of volatile organic compounds (VOCs) or not. Results obtained for both virgin GCLs are very close to each other. No impact of the nature of the bentonite could be evidenced. An increase in the diffusion coefficient was noticed when the GCL had experienced cation exchange while in contact with a synthetic leachate for DCM, DCA and TCE at a given bulk GCL void ratio. The largest increase, by a factor 2.6, was observed for TCE. However, this increase in diffusion coefficient could be balanced by a decrease in the bulk GCL void ratio from 3.9 to 3. The increase in the diffusion coefficient of VOCs thus does not seem to be of concern, for the range of GCL hydraulic conductivities generated through cation exchange in this study.