Lipoxygenase enzymes (LOXs) convert polyunsaturated fatty acids to organic hydroperoxides. This process is initiated by the transfer of a hydrogen atom from a fatty acid C-H bond to an active-site metal(III)-hydroxo unit. While LOXs from many organisms, including mammals and plants, utilize an iron cofactor, several fungal LOXs utilize manganese. To understand the geometric and electronic factors that control the hydrogen-atom abstraction step in these enzymes, and compare iron and manganese reactivity, our lab has generated MnIII-hydroxo and FeIII-hydroxo model complexes. We have combined crystallographic, spectroscopic, and computational methods to determine the geometric and electronic structures of these complexes. We then performed thermochemical measurements to determine the basicity of the hydroxo ligands and the O-H bond dissociation free energies for M(II)-aqua complexes. These thermochemical data are valuable, as hydrogen-atom transfer reactions often show strong correlations between reaction rate and driving force. We explored the reactivity of the MnIII-hydroxo and FeIII-hydroxo model complexes in hydrogen-atom transfer reactions using a variety of substrates. A combination of the kinetic and thermodynamic data allow us to identify factors that affect the hydrogen-atom transfer reaction rates.