Activated carbons (ACs) impregnated with triethylenediamine (TEDA) and quinuclidine (QD) are prepared for the removal of methyl iodide (CH3I) gas in the event of nuclear accidents. The effect of the tertiary amine loading (1, 5, and 10 wt.%) on the adsorption capability of the ACs is evaluated under dry and humid (RH 75%) conditions. Experimental evaluation of the 'breakthrough' combined with rigorous first-principles calculations enables us to identify the fundamental mechanism underlying the superior adsorption capability of QD to TEDA for real application, especially under humid conditions. The adsorption of CH3I by the impregnated ACs under dry conditions strongly depends on the physical properties (porosity) of the adsorbent. On the other hand, the chemical properties and the tertiary amine loading play dominant roles in CH3I adsorption at 75% RH. In particular, this study suggests a novel mechanism of the protonation reaction, rather than the already known alkylation of tertiary amines, to capture CH3I chemically under humid conditions. The molecular-level behavior of amines and water is a key factor for the enhanced adsorption of methyl iodide. The adsorbents developed herein can be applied to the efficient removal of radioactive methyl iodide in severe accidents.