Abstract : Iodide sorption experiments were conducted on clay stone samples originating from the Callovian-Oxfordian formation under experimental conditions as close as possible to in situ conditions. The total natural iodine content of the formation is shown to be very constant throughout the formation, ranging from 2 to 3 ppm. This range is in agreement with a past iodine accumulation in the marine organic matter of the sediment before and during deposition, and early diagenesis. At variance with total iodine, the leached iodine concentrations are variable. If leached iodine is considered to represent porewater solute iodine, its concentration can be calculated and ranges from 0 (below detection limit) to ~60 µmol/L and represents 0 to 25 % of the total iodine. The reason for this variability is not understood. Sorption isotherms were determined either for natural 127I- solutions or for 131I- spiked 127I- solutions, with concentrations ranging from 10-9 to 10-3 mol/L at solid to liquid ratios from 10 to 200 g/L. No or little sorption was encountered, Kd values being in the range 0-0.5 L/kg with statistical and analytical error bands being greater than the Kd values, with the exception of one experiment at low solid to liquid ratio (10 g/L), showing significant Kd values of ~25 L/kg. In sorption experiments with natural 127I- and at the lowest added iodide concentrations (< 10-6 mol/L) an apparent negative Kd was obtained due to the iodide content in the solid porewater that was leached once the solid was suspended. The low affinity of iodide for argilite is thus confirmed. However, based only on these results and given the extent of the error bands, one cannot discard a limited iodide uptake. Literature data on iodide diffusion on similar rock materials have already shown that iodide does not behave like chloride. The retention mechanism of radio-iodide is discussed in the light of the present results and diffusion data. A model involving isotopic exchange between the natural iodine content of the geological formation and radio-iodine allows all of the results to be described. Not all the iodine in the formation appears to participate in isotopic exchange reactions with the solution. A quantification of the isotopically labile fraction of iodine would allow the effect of isotopic exchange on radio-iodide migration throughout the Callovian-Oxfordian formation to be assessed and predicted.