Adsorption of trichloroethylene (TCE) by activated carbon preloaded with humic and fulvic acids was studied under several conditions in completely mixed batch systems. The authors investigated how molecular weight and molecular-weight distribution of preloaded humic substances affected subsequent adsorption of TCE. TCE isotherms were measured on preloaded carbon in completely mixed batch reactors (CMBRs) and compared with TCE adsorption on nonpreloaded carbon. TCE isotherms were modeled using the two-parameter Freundlich and the three-parameter Langmuir-Freundlich isotherms. The effects of preloading and of molecular weight were evaluated in terms of changes in the TCE isotherms and computed site-energy distributions.