Iodine is one of the most problematic radioisotopes in the context of nuclear waste geological disposal due to its high mobility. Considerable effort has been dedicated to the measurement of its potential retardation during diffusive transport leading to conflicting results, from no retardation to significant retardation, leading in turn to considerable debate. The present study aims at providing new insights into this aspect of the iodine problem by careful quantification of iodine reservoirs in the Callovian-Oxfordian (Cox) clay rock taken here as model material for these studies. The present study confirmed the ubiquitous presence of iodine at 1-5 mg kg(-1) level in the Cox clayey formation. The iodide concentration level in the pore-water is also confirmed at a value in the range similar to 20-40 mu mol L-1, i.e. higher than the expected range of radio-iodine concentration in the far-field of the storage. Surprisingly, most of the iodine was found not to be associated with organic matter but rather in an inorganic form associated with carbonate minerals. This result has potentially significant implications for the fate of radio-iodine. In undisturbed far-field conditions, most natural iodine would not be accessible for isotopic exchange with radioactive iodine, reducing the effective Kd to negligible values. During laboratory experiments, good monitoring of the geo-chemical parameters (at least the Eh, pH, P-CO2, [Ca] and [Mg]) is mandatory to avoid iodine-bearing carbonate precipitation and to enable rigorous interpretation of the iodide diffusion/retention experiments.