Abstract This study is concerned with the absorption properties of a fractionated humic substance at varying pH's on surface-treated carbon adsorbents. Activated carbon fiber (PF) made from phenol resin, Pittsburgh activated carbon (AC), and electronic conductive carbon black (EC) are used. The adsorption properties of the carbon differ greatly depending on the characteristics of the humic substance fractions. The humic substance used is divided into two fractions of different molecular weights in the neutral solution. The lower- (mol wt 100 to 1000 as polyethylene glycol (PEG)) and higher- (mol wt 1000 to 5000 as PEG) molecular-weight components are adsorbed well on PF and AC, having a large volume of micropores, and EC, being rich in mesopores, respectively. After the removal of surface polar substances from the adsorbents, we correlate the adsorptive capacity and the pore-size distribution. These results suggest that pore-size distribution is an important factor in removing humic substances effectively.