To enhance our understanding of trace element partitioning at oxide-water interfaces, we studied the local coordination environment of cadmium(II) and selenite complexes sorbed on aluminum oxides, in situ, using X-ray absorption spectroscopy (XAS). The adsorbents included porous, high surface area transition aluminas (ALCOA CP-5 and C-33) and the minerals corundum (alpha-Al2O3) and gibbsite (gamma-Al(OH)(3)). Cadmium sorption densities ranged from 1.2 to 12.0 mu mol/m(2) and selenite sorption densities ranged from 1.2 to 4.6 mu mol/m(2). X-ray absorption fine structure (XAFS) analysis of the selenite spectra indicates that Se is coordinated to three O atoms at 1.69 Angstrom, regardless of substrate, and that the complexes are mononuclear. XAFS analysis of low sorption density cadmium complexes suggests that cadmium is coordinated to six oxygens at 2.33 Angstrom and that the complexes are mononuclear. Analysis of a high total cadmium concentration sample (10(-3) M) gave O first neighbors at 2.35 Angstrom and Cd second neighbors at 3.84 Angstrom, suggesting the formation of a disordered cadmium hydroxide or cadmium hydroxocarbonate precipitate. Absence of Al second neighbors In the selenite and the low sorption density cadmium samples is probably caused by the low backscattering amplitude of Al and thermal and static disorder effects. These results, in combination with sorption isotherm data, suggest that, under the conditions studied, cadmium and selenite diffuse into the pores of the transition aluminas and sorb as mononuclear complexes. These results have significant implications for the fate of trace elements in subsurface environments and the remediation of waters and groundwaters.