We have used X-ray absorption fine structure (XAFS) spectroscopy to investigate Cu(II)sorption complexes on high surface area amorphous silica (am-SiO2) and gamma-Al2O3. For Cu(II) on gamma-Al2O3, analysis of XAFS data collected after a solution-solid total contact time of 80-170 h showed that monomeric Cu(II) species predominate at surface coverages of 0.007 and 0.05 mu mol m(-2). Cu(II) on the gamma-Al2O3 surface has aluminum second neighbors at about 2.8 a. Geometrical considerations indicate that this distance is consistent with models of Cu(II) binding to the gamma-Al2O3 surface in inner-sphere bidentate or monodentate modes on Al(O,OH)(6) octahedra. For Cu(II) sorbed on am-SiO2, analysis of XAFS data collected after a solution-solid total contact time of 80-110 h showed that a dimeric Cu(II)-surface complex predominates, along with a minority monomeric Cu(II) surface species, at 0.03 and 0.05 mu mol m(-2). The XAFS-derived Cu-Si distance is in the range of 2.98 to 3.05 Angstrom, suggesting that Cu(II) binds to am-SiO2 in an inner-sphere, monodentate fashion. XAFS spectra of Cu(II) sorbed on am-SiO2 collected after 20-30 h total contact time are quantitatively different from those collected after 80-90 h contact time. Analyses of these spectra indicate that the ratio of dimeric to monomeric Cu(II) surface complexes has : increased with contact time. A discussion of the processes responsible for the different Cu(II) complexes on am-SiO2 and gamma-Al2O3 is presented.