Activated carbon adsorption is an widely used process in environmental engineering. Alkaline impregnated activated carbon has been used to enhance the adsorption capacity for odorous compounds from gas streams. This study investigated the physicochemical and adsorption characteristics of one virgin and four alkaline-impregnated activated carbon samples. The four alkaline additives were NaOH, Na2CO3, KOH, and K2CO3 and the impregnated activated carbons were referred to as NaOH-IAC, Na2CO3-IAC, KOH-IAC, and K2CO3-IAC. The specific surface area, micropore area, and micropore volumes were reduced in the impregnated activated carbon systems. The adsorption capacity of H2S and CH3SH increased. This indicated that the physical properties were not the predominant influence on adsorption behavior. The impregnated activated carbons were ranked NaOH > Na2CO3 > KOH > K2CO3 for H2S adsorption. The NaOH-IAC demonstrated 3.2 and 2.2 times the adsorption capacity for H2S and CH3SH, respectively, compared to the virgin AC sample. Increasing the vacuum and immersion duration increased the alkaline quantity of NaOH impregnated on activated carbon. The NaOH-IAC(50) (50 mg NaOH/g carbon) sample performed the best. It had 6.9 times the adsorption capacity of the virgin AC. The humidity that coexisted in the H2S and CH3SH gas streams enhanced the H2S and CH3SH adsorption capacity of NaOH-IAC. At 50% relative humidity and 50 ppm H2S, the NaOH-IAC sample exhibited the maximum adsorption capacity for H2S. This carbon attained 30.3 times more capacity than the virgin AC.