There is an urgent need to develop materials and processes that reduce the energy penalty associated to the CO2 capture step. Biochars are appealing adsorbents for post-combustion CO2 capture applications due to their low cost, stability in moisture conditions and microporous nature. Series of carbon adsorbents were prepared from almond shells and olive stones by single-step activation with air at 400–500 °C, and with lower O2 concentration in the activating gas, 3–5%, at higher temperatures (500–650 °C). This process entails energy savings compared to conventional activation with carbon dioxide or steam. It has been found that the pore size distribution can be tailored by adequately selecting the activating conditions. Carbons obtained under lower oxygen partial pressures and higher temperatures present narrow microporosity, which is essential for the adsorption of CO2 at low partial pressures. These appealing low-cost adsorbents have competitive CO2 working capacities and high CO2/N2 equilibrium selectivity in conditions that can be considered representative for post-combustion CO2 capture, thus showing potential for this application. Work carried out with financial support from the Spanish MINECO (Project ENE2011-23467), co-financed by the European Regional Development Fund (ERDF). M.G.P. acknowledges funding from the CSIC (JAE-Doc program), and A.S.G. acknowledges a contract from the MINECO (FPI program); both programs are co-financed by the European Social Fund. Peer reviewed