Whereas the amount of cyclodextrin (CD) adsorbed onto the large-pore activated carbon A (AC-A) increased with the number of glucose units, the amount adsorbed onto the small-pore activated carbon B (AC-B) showed the opposite tendency. This behavior can be accounted for in terms of a molecular exclusion. It is known that a good linear relationship is obtained between the Freundlich constants log K and 1/N for hydrophobic adsorption. The adsorption of CDs onto AC-A obeyed this relation, but, because of the molecular exclusion, the plots of AC-B deviated greatly. The adsorption of CDs onto AC-A was not explainable in terms of solubility. This could be because, in the case of a solid compound, adsorbability depends on the chemical potential of the molecule in aqueous solution whereas solubility depends also on the heat of fusion of the solid. In order to estimate the relative chemical potential of CDs in water, a method based on the numbers of carbon atoms and oxygen atoms in the molecule was devised which allowed a more accurate estimation of CD adsorbability than did solubility. The mean pore diameter of AC-A increased after CD adsorption, while that of AC-B showed little change.