Sorption of oxyanions to soil minerals significantly impacts their transport behavior and fate in soil systems. The precise nature of how an oxyanion sorbs to a hydrated mineral surface is of fundamental importance and aids improvement of predictive surface complexation models. Recently, computational chemistry has been used to investigate soil oxyanion sorption mechanisms; particularly to the interpretation of extended X-ray absorption fine structure and Fourier transform infrared (FTIR) spectroscopic data. Computational studies have been limited to hybrid molecular orbital/density functional theory (MO/DFT) or pure DFT methods applied to small cluster models. Using MO/DFT calculations to aid interpretation of attenuated total reflectance FTIR spectroscopic data, Paul et al. (2005) investigated the effect of dehydration on sulfate adsorption at the Fe-(hydr)oxide-water interface. MO/DFT IR frequency calculations on cluster models revealed that sulfate adsorption on hydrated Fe-(hydr)oxide surfaces results in monodentate and/or bidentate bridging complexes under most experimental conditions. However, as the surface dehydrates a significant fraction of adsorbed sulfate protonates to form bisulfate. MO/DFT calculations indicated that the speciation change is likely reversible. The implications of this work to soil systems are important because soils are constantly subjected to wetting and drying. How wetting and drying cycles influence soil oxyanion sorption/desorption mechanisms is poorly understood. The purpose of this study was to investigate sulfate adsorption to Fe-(hydr)oxide surfaces using two independent computational approaches. Results from MO/DFT cluster models will be compared to periodic ab initio DFT calculations performed on specific surfaces of Fe-oxides. The periodic ab initio DFT calculations are performed on more realistic surfaces and have several advantages over MO/DFT cluster models. This study will compare and contrast the predicted geometries of adsorbed sulfate complexes on Fe-(hydr)oxide surfaces from the perspective of MO/DFT cluster and periodic ab initio DFT methodologies.