The aim of this study is to investigate the effect of moisture on adsorption efficiency of hydrogen sulfide (H2S) by impregnated activated carbon (IAC). Copper(II) nitrate was used as an impregnant. Two humidification conditions of IAC, pre-moistened and gas stream containing moisture, were studied. The experimental results revealed that the copper species onto the IAC was suggested to be Cu(OH)(2) that deposited on activated carbon during the impregnation process.. The adsorption mechanism of H2S by copper impregnated IAC was proposed, involving physic-sorption and chemical reactions. Moreover, the H2S breakthrough capacity decreased with increasing the relative humidity of gas stream. The causes were attributed to three points as follow: the competition adsorption occurred between moisture and H2S; the copper(II) species reduced to copper(I) species leading to IAC deactivation; and the rate of chemical reaction restrained by moisture. (c) 2006 Elsevier B.V. All rights reserved.