CO2 separation from synthesis gas or flue gas by activated carbon (termed AC) has been widely used or received much attention in recent years. Due to the different origin and preparation methods of ACs, their physico-chemical properties are usually significantly different, including pore structure, surface properties and ash. Which parameter influences CO2 adsorption and to what extent are still controversial in the literature. This work presents a systematical study in this aspect under conditions of pressure swing adsorption (PSA, 273.0-293.0 K and P-CO2 of 0.01-0.10 MPa). Results indicate that the surface properties of AC, exactly oxygen-containing functional groups, have little effect on CO2 adsorption, while the volume of ultra-micropore (<0.7 nm) has a significant effect on excess CO2 adsorption capacity, showing a linear relationship at PSA conditions. The ash in biomass-derived AC, mainly K compound, has a positive effect on CO2 adsorption while the ash in coal-derived AC, mainly Ca, Si, Al and Fe compounds, are inert to CO2 adsorption. (C) 2013 Elsevier B.V. All rights reserved.