Adsorption equilibria of eighteen kinds of organic compounds in their aqueous solution were measured on activated carbon and synthetic adsorbents, Affinity coefficients for adsorption were determined by applying the DR equation to measured isotherms. It was found that the affinity coefficient depended on substituent groups of aromatic compounds. The adsorptive interaction on the activated carbon and the synthetic adsorbents was studied based on the frontier orbital theory, Electronic states of adsorbent, adsorbate and solvent were calculated using the semiempirical molecular orbital method, Then, the characteristic energy of adsorption in aqueous solution was estimated, Affinity coefficients were correlated by the characteristic energy ratio to the energy of the reference adsorbate, It was found that charge transfer interaction played an important role in the adsorption of aromatic compounds in aqueous solution. A correlation procedure of adsorption isotherm was proposed based on the DR equation and the characteristic energy.