In this paper we present results that describe the speciation of Cu sorbed to the clay mineral vermiculite, with special attention to the effects of reduction on Cu sorption complexes. Sorption complexes were studied using powdered extended X-ray absorption fine structure (EXAFS) spectroscopy, polarized EXAFS spectroscopy, and X-ray diffraction (XRD). Ionic strength (I), background ion, and clay reduction (structural iron) were varied in the sorption samples. At low I EXAFS results indicate that Cu was surrounded by up to six water molecules sorbed in the interlayer of reduced vermiculite. EXAFS results from Cu-equilibrated reduced vermiculite with high I Ca background electrolyte revealed that Cu was surrounded by 4 O atoms at 1.95 angstrom and 8 second shell O atoms at 3.14 angstrom. Angular dependence of the second shell O atoms interpreted from the polarized-EXAFS spectra indicated that the atoms are out of plane from the basal plane of the vermiculite (inclined similar to 25 degrees from the ab plane). The local atomic environment and angular dependence of the EXAFS spectra suggest that the Cu atorns are adsorbed above the hexagonal cavities of the reduced clay mineral and form a Cu dimer in the interlayer. This adsorption mechanism was not observed in the non-reduced vermiculite under identical equilibration conditions. Our results provide molecular level evidence that Cu sorption mechanisms on vermiculite are dependent on solution conditions, such as redox potential and background electrolyte. These results can be used to develop better models of Cu sorption mechanisms on clay mineral surfaces. (D 2005 Elsevier Inc. All rights reserved.