<p>Carbon dioxide (CO₂) adsorption at high pressures by alkali-impregnated activated carbons was studied in this paper. Four types of activated carbon, prepared by the two-step activation method and the activation combined with oxidation method, were impregnated in different concentrations of NaOH solution of 1, 4, 7 and 10% by weight. The results of CO₂ adsorption at 0&#xa0;°C up to the saturation pressure showed that the maximum adsorption capacity was obtained from the activated carbon prepared by the activation combined with oxidation method under conditions of 180&#xa0;min of total activation time, two cycles of carbon oxidation and 1 weight% NaOH impregnation. An increase of the concentration of NaOH impregnation solution decreased the adsorbed amount of CO₂ for the four types of carbons used in this investigation. A Grand Canonical Monte Carlo (GCMC) simulation was used to investigate the adsorption mechanism of CO₂ in the finite-length slit pore model in the absence and presence of NaOH. An early onset in the adsorption isotherms can be observed in the heterogeneous pores. The alkali can enhance the adsorbed amount at low pressures, when pressures increase, it may cause difficult diffusion to the pore. The allocation of NaOH on carbon surfaces also affects the adsorption behavior. The adsorption isotherm for the fixed and random topologies with 1%weight NaOH can enhance the adsorption of CO₂ in the larger pore at high pressures too. While in the case of smaller pore at high pressures, the fixed topology showed the domination adsorption isotherm than the homogeneous and the random topology pores.</p>