The competitive sorption behavior of Cu2+ and propisochlor on humic acids (HA) was investigated using a batch method. The sorption equilibrium time of propisochlor on HA was 12 h when its initial concentration was 4 or 10 mg L-1. The results showed that the Langmuir model can best describe the sorption behavior of propisochlor on HA. The added Cu2+ reduced the solid-phase concentration of propisochlor on HA, and the Langmuir model can also describe the sorption behavior of propisochlor on HA when the Cu2+ concentration is 100 and 200 mg L-1. The sorption of propisochlor on HA did not remarkably affect the sorption of Cu2+ on HA when the concentration of propisochlor was below 20 mg L-1. Cu2+ may compete with propisochlor for the sorption sites of HA, such as carboxylic and phenolic groups. It can be concluded that Cu2+ fast adsorbed to the HA matrix, altered its physical and chemical properties, and thus decreased the solid-phase concentrations of propisochlor on HA. In natural water, Cu2+ may promote the release of propisochlor from HA, and thus affect its transport, transformation, and fate in the environment. (c) 2005 Elsevier Inc. All rights reserved.