Previously, we showed that sorption of anthropogenic organic compounds in soils is competitive as a result of the heterogeneous sorption potential of soil organic matter (SOM). We hypothesize that naturally occurring, low molecular weight organic molecules or ions could compete with anthropogenic molecules for sorption sites in SOM. We show here that natural aromatic acids suppress sorption from water of 1,3-dichlorobenzene (1,3-DCB) or 2,4-dichlorophenol (2,4-DCP) by up to similar to 40% to two soils, a Pahokee peat (93% SOM) and a Cheshire fine sandy loam (3% SOM). The experiments were performed at the natural pH of the soil suspension (5.0 and 5.6, respectively), at which the aromatic acids were mostly ionized. In a screen test at 100 mu g/mL aromatic acid concentration, 10 out of 11 competed significantly with 2,4-DCP. Five were selected for further study: vanillic, p-hydroxybenzoic, trans-cinnamic, p-coumaric, and ferulic acids. The selected five aromatic acids as well as the two anthropogenic compounds gave nonlinear isotherms (Freundlich exponent, 0.507-0.852), indicating a distribution of site energies in SOM. The pattern in the competitive effect among the aromatic acids was similar for the two soils; since the peat soil is almost mineral-free, this indicates that competition occurred in the SOM phase. In nearly all cases, suppression of sorption decreased exponentially with increasing aromatic acid concentration, as would he expected for direct competition for sites. These results are in accord with the dual-mode model of sorption to SOM we proposed earlier, in which both solid-phase dissolution and hole (site)-filling mechanisms take place and in which competition takes place only in the hole-filling domain. The results show that aromatic acids and possibly other small natural organic molecules can occupy and block adsorption sites in SOM, potentially affecting the fate, transport, and bioavailability of anthropogenic organic compounds in the environment.