The Freundlich isotherm has been widely used in the design of activated-carbon adsorption processes. The isotherm is easy to use and is applicable to a wide spectrum of organic compounds and adsorbents. The main drawback for the isotherm is that it is an empirical formula requiring experiments to determine its coefficients. To alleviate this drawback, a procedure is developed in this study to correlate the Freundlich coefficients-with the basic properties of three components involved in adsorption (adsorbate, adsorbent and solvent). Chloro-and nitrophenols were used as the test adsorbates, and granular activated carbon (GAG) was used as the adsorbent. The isotherm data showed that the percentage of the GAC pore surface covered by phenolic molecules was a better measurement for the amount adsorbed than the traditional mass-based solid concentration. A solution concentration normalized with respect to the solubility of the phenolics was used to account for the effects of phenol-water interactions. Isotherms of surface coverage versus normalized concentration conformed very well to a modified Freundlich model. The modified Freundlich exponent (l/n') was found to have an inverse linear relationship with the electron density of phenolics calculated from molecular orbital theory. The correlation will allow the prediction a priori of l/n' from the molecular structures of the adsorbate and adsorbent. (C) 1997 Elsevier Science B.V.