Manganese-hydroxo species have been implicated in C-H bond activation performed by metalloenzymes, but the electronic properties of many of these intermediates are not well characterized. The present work presents a detailed characterization of three Mn-n-OH complexes (where n = II, III, and IV) of the tris[(N'-tert-butylureaylato)-N-ethylene]aminato ([H(3)buea](3-)) ligand using X- and Q-band dual mode electron paramagnetic resonance (EPR). Quantitative simulations for the [Mn(II)H(3)buea(OH)](2-) complex demonstrated the ability to characterize similar Mn-II species commonly present in the resting states of manganese-containing enzymes. The spin states of the Mn-III and Mn-IV complexes determined from EPR spectroscopy are S = 2 and 3/2, respectively, as expected for the C-3 symmetry imposed by the [H(3)buea](3-) ligand. Simulations of the spectra indicated the constant presence of two Mn-IV species in solutions of [Mn(IV)H(3)buea(OH)] complex. The simulations of perpendicular- and parallel-mode EPR spectra allow determination of zero-field splitting and hyperfine parameters for all complexes. For the Mn-III and Mn-IV complexes, density functional theory calculations are used to determine the isotropic Mn hyperfine values, to compare the excited electronic state energies, and to give theoretical estimates of the zero-field energy.