Ultramicroporous carbon materials with uniform pore size accurately adjusted to the dimension of electrolyte ions or CO2 molecule are highly desirable for maximizing specific capacitance and CO2 uptake. However, efficient ways to fine-tuning ultramicropore size at angstrom level are scarce. A completely new approach to precisely tuning carbon ultramicropore size at sub-angstrom level is proposed herein. Due to the varying activating strength and size of the alkali ions, the ultramicropore size can be finely tuned in the range of 0.60-0.76 nm as the activation ion varies from Li+ to Cs+. The carbons prepared by direct pyrolysis of alkali salts of carboxylic phenolic resins yield ultrahigh capacitances of up to 223 F g(-1) (205 F cm(-3)) in ionic liquid electrolyte, and superior CO2 uptake of 5.20 mmol g(-1) at 1.0 bar and 25 degrees C. Such outstanding performance of the finely tuned carbons lies in its adjustable pore size perfectly adapted to the electrolyte ions and CO2 molecule. This work paves the way for a new route to finely tuning ultramicropore size at the sub-angstrom level in carbon materials.