The effect of humidity on the adsorption and desorption of methyl iodide was investigated in a TEDA (triethylenediamine)-impregnated activated carbon bed. The amounts of chemisorption and physical adsorption of methyl iodide were measured in different humidity levels for base and TEDA-impregnated activated carbons. The physical adsorption of methyl iodide in the presence of water vapor was fitted with the potential theory-based Dubinin-Raduahkevich equation. The significant amount of chemisorption even in high humidity conditions confirmed the effectiveness of TEDA-impregnation for trapping methyl iodide permanently.