Activated carbon fibers, which exhibit high specific area and numerous active surface sites, constitute very powerful adsorbents and are widely used in filtration to eliminate pollutants from liquid or gaseous effluents. The fibers studied in this work are devoted to the filtration of gaseous effluent containing very small amounts (few vpm) of hydrogen sulfide. Preliminary experiments evidenced that these fibers weakly adsorb hydrogen sulfide. To improve their fixation capacity toward H2S the activated fibers are impregnated in an aqueous solution of potassium hydroxide. The impregnation treatment usually takes place before activation but in this work it occurs at room temperature after activation of the fibers. A further thermal treatment is performed to increase the efficiency of the system. The overall treatment leads to the creation of basic sites showing a great activity for H2S gas in the presence of water vapor. The mechanism has been established by a series of characterizations before, during, and after the different operation units. The KOH deposited after impregnation is carbonated into KHCO3 at room temperature and then decomposed into K2CO3 during the thermal treatment. K2CO3 and H2S dissolve in a liquid aqueous solution formed on the fiber surface. Then carbonate ions and H2S molecules react together almost completely to yield HS− species. As a consequence the sorption capacities of hydrogen sulfide on the impregnated fibers are much higher, even for small hydrogen sulfide volume fractions.