CO 2 adsorption capacity of a commercial activated carbon was improved by using HNO 3 oxidation, air oxidation, alkali impregnation and heat treatment under helium gas atmosphere. The surface functional groups produced were investigated by diffuse reflectance infrared Fourier transform spectrometer (DRIFTS). CO 2 adsorption capacities of the samples were determined by gravimetric analyses for 25–200°C temperature range. DRIFTS studies revealed the formation of carboxylic acid groups on the HNO 3 oxidized adsorbents. Increased aromatization and uniform distribution of the Na particles were observed on the samples prepared by Na 2 CO 3 impregnation onto HNO 3 oxidized AC support. The adsorption capacities of the nonimpregnated samples were increased by high temperature helium treatments or by increasing the adsorption temperature; both leading to decomposition of surface oxygen groups, forming sites that can easily adsorb CO 2 . The adsorption capacity loss due to cyclic adsorption/desorption procedures was overcome with further surface stabilization of Na 2 CO 3 modified samples with high temperature He treatments. With Na 2 CO 3 impregnation the mass uptakes of the adsorbents at 20bars and 25°C were improved by 8 and 7 folds and at 1bar were increased 15 and 16 folds, on the average, compared to their air oxidized and nitric acid oxidized supports, respectively.