Hydrophobic organic contaminants in aquatic systems bind to sediments or to suspended particles, and these associations affect the transport and bioavailability of contaminants and thus alter the exposure received by biota. We present evidence demonstrating the importance of another less obvious natural sorbent—dissolved organic macromolecules (DOM), such as dissolved humic materials—in altering the environmental fate of contaminants. Binding of a polycyclic aromatic hydrocarbon, benzo[a]pyrene, to DOM and to particles was measured for each sorbent and in a mixed system containing both sorbents. DOM competed with particles for binding of the dissolved contaminant and reduced the amount bound to particles. Binding to each sorbent was independent and noninteractive. Based on these results, equations were developed to indicate those environments and ecosystems for which failure to account for the role of DOM as a sorbent would result in 50 or 90% errors in steady state predictions about the exposure of aquatic organisms to contaminants.