Abstract: Commercial coconut shell based activated carbon (AC) was used to investigate the production of increased mesoporosity by impregnating with catalytic metal salts such as Fe(NO3)3 and Fe2(SO4)3 followed by CO2 gasification. The salts are found be effective in enlarging the pore size of AC and Fe(NO3)3 is the more effective one to increase pore size and surface area, up to 10nm and 1930m2/g, respectively. Modified AC shows a much larger adsorption capacity (5-8 times as large as that of as-received AC) and a more rapid adsorption rate for vitamin B12. Three mechanisms are proposed to modify the surface properties and pore size of AC: (1) NOx and SOx liberated from the impregnated salts burn the carbon wall to enlarge pore size; (2) Metal oxide reacts with the carbon wall to enlarge pore size, accompanied by reduction to the corresponding metal; and (3) Metal has a catalytic effect during CO2 gasification of the carbon wall of the AC. Graphitization of the carbon wall caused by metal is also observed. The present modification method is a promising, convenient and cheap way to modify AC to obtain mesopores.