Attenuated total reflectance FTIR spectroscopy was used to characterize Al(III)-acetate complexation at 25 degrees C, in homogeneous aqueous solution and at the water-aluminum hydroxide interface. The data collected in aqueous solution, at [Al](tot) = 0.080 M, [OAc](tot) = 0.040 M, and pH < 4.3, indicate the presence of only one dominating complex which, by spectral analysis, is shown not to involve monodentate acetate coordination but rather a syn-syn bridging geometry. In quantitative terms, the data strongly support the appearance of a binuclear mixed-hydroxo species [Al-2(OH)(2)OAc](3+) which, in view of the above, probably consists of a dihydroxo-bridged Al-2(mu-OH)(2)-unit to which the acetate ion bridges via the apices of the two Al(III) octahedra. Also at the water-aluminum hydroxide interface, only one dominating surface complex is indicated. Zn contrast to the aqueous species this complex is a weak mononuclear outer-sphere complex, and it is speculated that, due to a lack of structural flexibility of the surface Al(III) octahedra, the formation of a bridging inner-sphere complex is prohibited. Based on the concept of surface complexation, and utilizing the extended constant capacitance model to account for contributions from electrostatic forces, an equilibrium model, which quantitatively describes these interactions in an ionic medium of 0.1 M NaCl, is presented. Copyright (C) 1998 Elsevier Science Ltd.