PURPOSE: Active carbon is deposited with a given amount of H2SO4 to provide an adsorbent of high capcity and durability for removing basic gas such as NH3 and the like.

CONSTITUTION: Active carbon prepared from coconut shell, coal or petroleum pitch is immersed in aqueous H2SO4 solution, and deaerated by vacuum to cause H2SO4 solution to penetrate into pores of active carbon. The treated carbon is then dried. Desirable quantity of H2SO4 deposited onto active carbon after drying is 25 to 60 wt%, or more preferably 30 to 40wt%. The resultant adsorbent is capable of adsorbing NH3, trimethylamine and other basic gases at a high rate of removal, and keeps its capcity for an extremely long period of time