A series of sustainable activated carbons (ACs) with large surface areas and tunable pore sizes is synthesized from Cladophora cellulose and its chemically modified derivatives in a one-step physical carbonization/activation process. The molecular structure of the cellulose precursors and the carbonization/activation atmosphere (N-2 or CO2) significantly influence the pore structure of the ACs. When using oxidized cellulose and its further cross-linkages as the precursor, the ACs have a large volume of ultramicropores (pore diameter < 0.8 nm). Activation in CO2 results in ACs with surface areas up to 1241 m(2) g(-1). These ACs have a high CO2 uptake capacity (2.29 mmol g(-1) at 0.15 bar, 5.52 mmol g(-1) at 1 bar; 273 K) and a high CO2-over-N-2 selectivity (42 at 273 K). In addition, the capacity of the ACs to adsorb vapors of volatile organic compounds (VOCs) is remarkable, with values up to 0.97 mmol g(-1) at very low VOC concentrations (200 ppmv). The ACs have ultrahigh VOCs-over-N-2 selectivity up to 9.35 x 10(3) at 293 K for 0.02 vol%/99.8 vol% of benzene/N-2 mixture. It is anticipated that these ACs will be useful as sorbents for the postcombustion capture of CO2 and for indoor removal and direct air capture of various VOCs.