Binding energies, intermolecular distances, and partial charges from ab initio studies of SO42- + H2O were used to develop a molecular mechanics model for SO42- in water. Structural, dynamic, and thermodynamic properties for the resulting model used in condensed-phase simulations agree well with experimental estimates. Thirteen waters are found to be present in the first solvation shell, and the residence time of these waters has been calculated to be 23 ps. The model anion has a free energy of solvation relative to xenon of -275 kcal mol(-1), which compares well with the experimental estimate of -266 kcal mol(-1).