Activated carbons (ACs) are widely used within the ventilation networks of nuclear facilities to trap volatile iodine species. In this paper, the performances of various commercial activated carbons towards the trapping of γ-labelled methyl iodide were evaluated in semipilot scale under different R.H. according to normalized procedures. A combination between the retention performances and the physico-chemical properties as deduced from several techniques was performed to gain insights about the AC influencing parameters on γ-CH3I capture. Different trends were obtained depending on the impregnant nature and the studied conditions. A high sensitivity of KI/ACs towards water vapor was outlined. At R.H. = 40%. The enhancement of water uptake by KI/ACs as deduced from water adsorption experiments, leads to decrease the available microporosity for CH3I physisorption, inducing therefore the reduction of performances as a function of KI content at these conditions. At R.H. = 90%, the adsorption mechanism was found to be governed by isotopic exchange reaction since 90% of the microporosity was occupied by water molecules. Therefore, a slight increase of DF was obtained in these conditions. This sensitivity was found to be of a lesser extent for TEDA/ACs displaying the highest retention performances whatever the studied condition.