Sorption of alpha-endosulfan by kaolinite and montmorillonite alone and in the presence of sorbed and dissolved humic acid (HA) was investigated (pH 8 and 25 degrees C). Three types of HA, Elliot soil HA (EHA), Peat HA (PHA), and Summit Hill HA (SHHA), were used to represent typical humic substances found in soils. For sorption of HA by either mineral, Freundlich sorption coefficient (K(f)) values appeared to decrease in the order of EHA > PHA > SHHA, which followed increasing polarity (expressed as the O/C atomic ratio) and decreasing percent-carbon content. For both clays, sorption of alpha-endosulfan by the HA mineral complex was greater than for sorption by the clay alone. Sorption of alpha-endosulfan by the HA mineral complexes followed the same order as the K(f) of the HAs (EHA > PHA > SHHA). Based on the amount of HA adsorbed by each mineral, organic carbon partition coefficients (K(oc)) were determined for sorption of alpha-endosulfan by two of the HA mineral complexes. The value of K(oc) for alpha-endosulfan sorption was greater for kaolinite EHA than kaolinite SHHA. However, the opposite trend was found with the montmorillonite HA complexes. Montmorillonite appeared to sorb alpha-endosulfan and/or HA with higher affinity than kaolinite, which likely is due to its 2:1 layer structure and higher surface area. Sorption of endosulfan diol, a hydrolysis product, by the minerals was much less than the parent pesticide.