It is known that some inorganic anions can be adsorbed by variable-charge soils specifically, resulting in the lowering of the potential of the clay particle. Reasoning similarly, organic anions should also have such an effect. In this article, the effect of the anions of five low-molecular-weight (LMW) organic acids existing widely in soils on the potentials of two variable-charge soils was examined. The results showed that the presence of organic anions led to a decrease in potential. The effect of different anions on potential followed the order oxalate > citrate > malate > maleate > acetate. The effect increased with the increase in anion concentration and decreased with the increase in pH. The extent of the effect on different soils was apparently related to their-iron oxide content. The presence of organic anions also led to a decrease in the isoelectric point (IEP) of the soil. The IEPs of two soils in organic anion systems followed the order acetate > maleate > malate > citrate. No IEP was detected for the oxalate system. (C) 2004 Elsevier Inc. All rights reserved.