Activated carbon was impregnated with a metal halide, and adsorption and desorption characteristics of CO on the carbon were measured by fixed-bed runs. It was found that the impregnation of PdCl2 or CuCl effectively increases CO absorption. PdCl2/CuCl-impregnated carbons were characterized by N-2 adsorption, SEM, EPMA, and XPS. Adsorption isotherms of CO were also measured on these carbons, and the influence of the loading of impregnant on CO adsorption was experimentally elucidated. A selection procedure of impregnant was proposed based on the frontier orbital theory. The perturbation energy for molecular orbital mixing was estimated by the HOMO-LUMO interaction. CO adsorption on impregnated carbons was qualitatively interpreted using the perturbation energy, and the energy was regarded as an index of impregnant selection.