The adsorption abilities of commercial activated carbon after thermal and chemical treatment were investigated. The chemical character of carbon surfaces was determined by applying the thermogravimetric method, measurement of adsorption isotherms of water vapor, and the process of neutralization of surface functional groups. Total amounts of oxygen and nitrogen in all carbon samples were estimated by elemental analysis. Adsorption-desorption isotherms of benzene vapor were used to determine the parameters of the Dubinin-Radushkevich equation characterizing the porous structure of investigated carbons. These analyses show that chemical modification of carbon samples leads to strong differentiation of the chemical character of adsorbent surfaces; however, the porous structures show distinct similarity. In order to study the adsorption equilibria from liquid phase, the adsorption isotherms were measured for ternary mixtures of benzene, diethyl ketone, and n-heptane on all carbons. A strong effect of the chemical character of the carbon surface on liquid adsorption was found and discussed for all systems.