Clay minerals and soil organic matter (SOM) are generally considered to be the most important soil components in the sorption of aqueous phase organic contaminants.  During the past 25 years, much emphasis has been placed on the dominant role of SOM in sorption.  However, there is increasing evidence that many organic contaminants are substantially sorbed from bulk water by clays, often in excess of that by SOM when compared on a unit mass basis.  Nitroaromatic compounds (NACs) are an interesting case in point.  We have examined in detail the molecular scale mechanisms and forces leading to the very high affinities of smectite clays for NACs.  These studies have employed a complimentary set of investigatory approaches including detailed bulk phase sorption measurements, novel x-ray diffraction techniques, infrared spectroscopy, molecular simulation and thermodynamic measurements to reveal the molecular scale determinants of NAC sorption by smectite clays.  Our studies have revealed that NAC affinity for smectites is strongly dependent on the nature of the inorganic exchangeable cations.  Clays saturated with cations having low hydration energies (e.g. K⁺) have high affinities for NACs.  For smectites, these cation effects are well understood at the molecular level.  Sorption occurs primarily in the clay interlayers, and cation hydration determines the interlayer distance, size of adsorption domains and ability of -NO₂ groups to complex directly with exchangeable cations.  Lower clay charge densities result in larger adsorption domains that promote adsorption.  Partial solute dehydration afforded by the subaqueous clay interlayer environment is also energetically favorable.