TEDA and KI co-impregnated activated carbons (ACs) are widely used in the nuclear facilities to prevent volatile iodine species (namely CH3I) dissemination into the environment. However, some uncertainties still remain namely about the most influencing adsorbent parameters, as well as the involved adsorption mechanisms. In this talk, a specific attention will be devoted to better assess the influence of KI progressive impregnation on the obtained retention performances through different test methodologies and using various characterization techniques (XPS, SEM/EDS, N2 porosimetry, H2O porosimetry…). Different trends were obtained depending on the studied condition when considering CH3I retention efficiency as deduced from standardized protocols. A decrease of decontamination factors (DF) was observed as a function of KI content at R.H. = 40 % (Fig.1 (A)). This decrease was attributed to the enhancement of H2O interaction with activated carbons after KI impregnation. Therefore, the available microporosity for CH3I physisorption is reduced (Fig.1 (A)). At R.H. = 90%, the adsorption mechanism was found to be governed by isotopic exchange reaction, leading therefore to a slight DF increase after progressive KI incorporation. Better performances were nevertheless obtained after TEDA impregnation due to its high affinity with methyl iodide [1]. The beneficial effect of KI for CH3I removal was highlighted namely through breakthrough experiments performed towards γ-labelled and stable CH3I. Indeed, a progressive gain in adsorption capacities after KI incorporation until about 70% for 5%KI/AC, was evidenced (Fig. 1 (B)).