Soil derived dissolved organic mater (DOM) has been shown to form stable complexes with the herbicide napropamide. These complexes enhance the transport of napropamide through soil columns. It has also been shown that facilitated transport of napropamide is dependant on cultural practices, such as allowing the herbicide to dry on the soil after application. Facilitated transport of napropamide can also be predicted based on sorption characteristics. To further understand and predict the nature of the napropamide-DOM complex, effluent from soil columns was subjected to equilibrium dialysis in order to characterize the complex based on size. Two soils, one containing sewage sludge-derived organic matter (SS) and the other having only natural organic matter (NoSS), were treated with napropamide and allowed to dry, promoting complex formation. Soil columns were prepared by packing a layer of treated soil over a layer of untreated, dry, sieved soil. Both layers were brought to the same bulk density by gently tapping the sides of the column. The soil columns were irrigated, and the collected effluent was placed in dialysis chambers. Each chamber was fitted with a dialysis membrane with 1 of the following molecular weight cutoffs: 500, 1000, 5000, 10,000, and 25,000 Daltons. After equilibration, napropamide concentrations were determined on both sides of the membrane. The complex containing particles larger than the membrane cutoff was quantified based on the amount of napropamide unable to cross the membrane. In both the SS and NoSS soils, most of the complex which was transported through the columns had a molecular weight between 500 and 1000 Daltons. The SS soil formed larger complexes than the NoSS soil, and the SS soil had more total complex in the effluent than the NoSS soil.