Iodine-129 along with its sister halogen Cl-36, are the two most significant radionuclides contributing to the human dose from recent safety assessments of deep or near-surface geological disposal of nuclear waste (SKB, 1999; Andra, 2005). Since iodine is generally present in environments as an anion (either iodide or iodate), it is soluble in groundwater. However it can be retained by the organic matter present in soils and in geological materials (Whitehead, 1984). Many studies have shown that iodine was associated with humic substances under laboratory and natural conditions. The few studies that have investigated the functional groups and the type of bound involved in the iodine sorption in organic molecules have suggested that phenolic groups would have the highest affinity for iodine (Warner et al., 2000; Mercier et al., 2002). Here we assess the relation between the composition (elemental and molecular) of an organic material and its ability to sorb iodine. This relation would give information on the nature of the bounds involved in iodine sorption and on the sorption capacities of various types of organic matter : fresh terrestrial organic matter (highly oxygenated/oxidised) to thermal organic matter(highly aromatic/reduced). For this purpose, we process to the progressive “thermal denaturation” of an organic material in high-pressure autoclaves. The material used is a black peat sample from a boreal peat bog. The denaturation is conducted for 24 hours under pressure (700 bar) at several temperature : 150°C, 200°C, 250°C, 300°C, 350°C and 400°C) (Monthioux et al., 1986). Following this treatment, the chemistry of the material is then thoroughly analyzed: C, H, O contents, molecular composition of both the extractable organic fraction (saturated and aromatic hydrocarbons and polar compounds by GC-MS) and the insoluble organic matter (by THM (Thermally assisted Hydrolysis and Methylation) – GC-MS ), separated by organic solvent extraction. Spectroscopic characterisation is also carried out using Fourier Transformed Micro-Infrared Spectroscopy on extractable organic matter and insoluble organic fraction. These analyses (molecular and spectroscopic)should allow the identification of oxygenated groups (such as phenolic and polyphenolic groups) and aromatic structures and their evolution during the “thermal denaturation”. A sorption experiment is also carried out on the denatured material, for a fortnight at a 1/10 solid to solution ratio with a KI solution. At the end of the contact period, the solutions are extracted and analysed for total I content. The samples having sorbed the highest quantity of iodine are then analysed with TEM and EDXS to relate the sorption of I to the structure of the compounds.