Clay minerals may be an important unrecognized sorptive phase for dioxins in soils and clay deposits. Smectites, especially Cs-saponite, effectively adsorbed dibenzo-p-dioxin (DD) from water, reaching 0.8% (wt/wt). Adsorption was promoted by exchangeable cations with low hydration energies, and negative charge in the smectite arising from the tetrahedral siloxane sheets. X-ray diffraction measurements revealed that as DD loading increased to > or =8000 mg/kg the clay basal spacing increased abruptly from 12.3 to 15.2 A demonstrating DD intercalation. The 12.3 A spacing provides an interlayer distance that closely matches the molecular thickness of DD. In this configuration DD is essentially dehydrated as it interacts with the opposing hydrophobic siloxane sheets and with coplanar Cs+ via one of the dioxin ring oxygens. Ab initio calculations suggest that geometrical structures form at higher loadings in which intercalated DD molecules adopt a butterfly geometry sandwiched between dehydrated interlayer Cs+ and the siloxane surface, consistent with the 15.2 A spacing, wherein Cs+ interacts with dioxin ring oxygens and benzene ring pi-electrons. Fourier transformation infrared measurements confirm that adsorbed DD is present in orientations that are not parallel with the interlayer planar siloxane surfaces of smectite.