Freundlich equation is developed by correlating its coefficients of n and K with molecular properties of adsorbate and adsorbent involved in adsorption systems. Adsorption isotherm tests in aqueous solution were conducted using a conventional batch bottle technique, in which an activated carbon and a synthetic styrene-based resin were used as adsorbents. The adsorbates used in this study included phenol, four chlorophenols, and three nitrophenols. The parameter values obtained from Freundlich equation were related to two molecular properties computed from molecular orbital (MO) theory: highest occupied molecular orbital (HOMO) density representing the probability of finding an electron in pi-orbital, and molecular orbital energy (in eV). These properties can be computed based on the knowledge of adsorbate and adsorbent molecular structures. The results showed that experimentally determined Freundlich constant l/n is correlated with total HOMO electron density of adsorbate and adsorbent. It was also found that the value of K/n has a relationship with the molecular orbital energy between adsorbent surfaces and adsorbate molecules, DeltaEL. These correlations are significant because Freundlich coefficients n and K can be estimated expeditiously from easily accessible molecular information, possibly excluding costly and time-consuming laboratory tests. (C) 2004 Elsevier B.V. All rights reserved.