Abstract: In order to explore adsorption mechanisms of organic contaminants on clay minerals, adsorption behaviors of humic substances on smectite were studied by using several analytical techniques including liquid-state ¹H nuclear magnetic resonance spectroscopy, high performance size exclusion chromatography, scanning electron microscope, surface area analysis and Fourier transform infrared spectroscopy. The results showed that both fulvic acid (FA) and humic acid (HA) underwent fractionation during the adsorption process due to the different affinity for the functional moieties of humic substances on smectite. FA/HA molecules with large molecular size and weight were preferentially adsorbed on the mineral surfaces. The amount of HA was higher than that of FA was adsorbed on smectite via hydrophobic partitioning. After FA adsorption, specific surface areas and pore volumes of smectite decreased by 3.67m²/g and 0.005cm³/g, respectively, while a slight increase was observed after HA adsorption. The results indicated that HA with larger particle size was preferred to accumulate on the external surfaces of smectite, while FA was prone to block the pores of smectite. The increase in pH (0.44and 0.41) in smectite suspensions after FA and HA adsorption suggested that ligand exchange occurred and FA/HA-smectite complexes were formed. The increase in the equilibrium pH or the decrease in the ionic strength led to the reduction of FA/HA adsorption on smectite, which implied that hydroxyl ions in the aqueous solutions were likely to suppress ligand exchange between negatively charged FA/HA groups and the surface hydroxyl groups of smectite. The results also implied that electrolytes could reduce the electrostatic repulsion between smectite and FA/HA.