Dissolved organic matter (DOM) is ubiquitous in the environment and can strongly affect the fate, mobility, and bioavailability of hydrophobic organic contaminants (HOCs). Interactions between HOCs and DOM have been generally considered to be linear distribution, but nonideal sorption of HOCs by DOM was reported in the literature. Most of the previous discussion on nonlinear interaction between HOCs and DOM was focused on experimental artifacts. This study was designed to investigate the nonlinear interaction between HOCs and DOM using a dialysis method. Nonlinear sorption (with n values varying from 0.91 to 0.95 for phenanthrene, and from 0.76 to 0.89 for pyrene), desorption hysteresis (with TII significantly higher than 0 in all conditions), and significant competition between phenanthrene and pyrene were observed consistently at different pHs. Our results provided strong support for site-specific sorption of HOCs in DOM. Significant pH and DOM concentration influence on HOCs-DOM interaction was observed. Conformation changes of DOM molecules under different solution conditions were proposed to explain the interactions between HOCs and DOM.