The influence of natural organic matter (NOM) constituents on contaminant distribution coefficients was evaluated by determining the K-OC values of aromatic and aliphatic organic compounds (solutes) with clays modified with both aromatic- and aliphatic-rich organic constituents. The studied compounds consisted of naphthalene, phenanthrene, n-pentane, and 2,3,4-trimethylmethane; the solid samples comprised two clays with little organic content, kaolinite and Ca-montmorillonite. Two aliphatic surfactants and three aromatic dyes, sorbed to the clays, served as reference NOM constituents. For solutes of comparable water solubilities, the organic-carbon normalized distribution coefficients (K-OC) of the aliphatic solutes between sorbed aliphatic organic matter and aqueous solution slightly exceed those of the aromatic solutes. By contrast, the aromatic solutes exhibited higher K-OC values than did the aliphatic compounds with sorbed aromatic-rich organic matter. The difference in K-OC values could be attributed to either comparable solubility parameters or the difference in the chemical structure between nonionic organic solutes and specific components of the simulated NOM. The much higher K-OC values observed for the aromatic solutes indicate that the NOM composition is a major factor determining the NOC environmental distribution. © 2005 Elsevier Inc. All rights reserved.