Amphiphilic molecules, such as humic substances and surfactants, are known to increase the apparent aqueous solubility of hydrophobic organic compounds (HOCs) in the aqueous phase because of their molecular structures, which consist of hydrophilic and hydrophobic moieties. In this study, we examined the effect of the structures of humic acid and HOCs on the sorption of four polycyclic aromatic hydrocarbons (PAHs) and an organochlorine pesticide, p,p'-DDT, to humic acid. As the number of aromatic rings was increased, the extent of solubility enhancement of PAHs by humic acid was increased. Although p,p'-DDT was more hydrophobic than pyrene in this study, the extent of solubility enhancement of p,p'-DDT by humic acid was lower than that of pyrene because of the molecular structures of the solutes. Anionic surfactants with and without aromatic rings also were studied for comparison, and the dianionic surfactant with two benzene rings exhibited similar results with humic acid, unlike the surfactants without and with one benzene ring. The results from this study indicate that bulky molecules, such as p,p'-DDT sorbed with more difficulty to the aggregates of amphiphiles with larger molecules, such as humic substances and the dianionic surfactants.