The adsorption isotherms of helium at 4.2 K were determined to elucidate the presence of ultramicropores in microporous solids such as activated carbons with different extents of activation. These isotherms were compared with the adsorption isotherms of nitrogen at 77 K. The micropore volumes determined from the cb analysis of helium adsorption agreed well with those of nitrogen adsorption except for the most highly activated carbon. Carbons with low-to-medium burnoff had no ultramicropores (pore width < 0.7 nm), but the presence of this porosity (inaccessible to nitrogen) was shown in the carbon with the highest burnoff. The Dubinin-Radushkevich (DR) and pore size distribution (PSD) analysis also supported this finding.