The effects of the adsorption of pyromellitate, an analogue for natural organic matter, on the dissolution behavior of corundum (alpha-Al2O3) have been examined over a wide range of pyromellitate concentrations (0-2.5 mM) and pH conditions (2-10). The adsorption modes of pyromellitate on corundum have first been examined using in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and are shown to be dominated by a fully deprotonated, outer-sphere pyromellitate species (equivalent to A1OH(2)(+)center dot center dot center dot Pyr(4-)) at pH >= 5.0. At lower pH conditions, however, an additional protonated outer-sphere species (equivalent to A1OH(2)(+)center dot center dot center dot H(2)Pyr(2-)) and an inner-sphere species are also evident. In accordance with the ATR-FTIR findings, modeling of macroscopic pyromellitate adsorption data using an extended constant capacitance treatment was possible using two outer-sphere (equivalent to A1OH(2)(+)center dot center dot center dot Pyr(4-) and equivalent to A1OH(2)(+)center dot center dot center dot H(2)Pyr(2-)) and one inner-sphere (A1Pyr(3-)) adsorbed pyromellitate species. The presence of adsorbed pyromellitate strongly inhibited the dissolution of corundum under acidic (pH < 5) conditions, consistent with a mechanism previously proposed by Johnson et al.(I) whereby outer-spherically adsorbed Pyr(4-) species sterically protect dissolution-active surface sites from attack by dissolution-promoting species such as protons. A reduction in the protolytic dissolution rate of corundum results. A reference Suwannee River fulvic acid, which also adsorbs to aluminum (oxyhydr)oxide surfaces in a predominantly outer-sphere manner, was similarly shown to strongly inhibit the dissolution of corundum at pH = 3.