Microporous carbon materials with extremely small pore size are prepared by employing polyaniline as a carbon precursor and KOH as an activating agent. CO(2) sorption performance of the materials is systematically investigated at the temperatures of 0, 25 and 75 °C. The prepared carbons show very high CO(2) uptake of up to 1.86 and 1.39 mmol g(-1) under 1 bar, 75 °C and 0.15 bar, 25 °C, respectively. These values are amongst the highest CO(2) capture amounts of the known carbon materials. The relation between CO(2) uptake and pore size at different temperatures is studied. An interesting and innovative point that the micropores with pore size smaller than a critical value play a crucial role in CO(2) adsorption at different temperatures is demonstrated. It is found that the higher the sorption temperature is, the smaller this critical value of pore size is. Pores smaller than 0.54 nm are manifested to determine CO(2) capture capacity at high sorption temperature, e.g. 75 °C. This research proposes a basic principle for designing highly efficient CO(2) carbon adsorbents; that is, the adsorbents should be primarily rich in extremely small micropores.