The anisotropic g and hyperfine tensors of the Mn di-mu-oxo complex, [Mn-2(III,IV)O-2(phen)(4)](PF6)(3)(CH3CN)-C-., were derived by single-crystal EPR measurements at X- and Q-band frequencies. This is the first simulation of EPR parameters from single-crystal EPR spectra for multinuclear Mn complexes, which are of importance in several metalloenzymes; one of them is the oxygen-evolving complex in photosystem II (PS II). Single-crystal [Mn-2(III,IV)O-2(phen)(4)](PF6)(3)(CH3CN)-C-. EPR spectra showed distinct resolved Mn-55 hyperfine lines in all crystal orientations, unlike single-crystal EPR spectra of other Mn-2(III,IV) di-mu-oxo bridged complexes. We measured the EPR spectra in the crystal ab- and bc-planes, and from these spectra we obtained the EPR spectra of the complex along the unique a-, b-, and c-axes of the crystal. The crystal orientation was determined by X-ray diffraction and single-crystai EXAFS (Extended X-ray Absorption Fine Structure) measurements. In this complex, the three crystallographic axes, a, b, and c, are parallel or nearly parallel to the principal molecular axes of Mn-2(III,IV)O-2(phen)(4) as shown in the crystallographic data by Stebler et al. (Inorg. Chem. 1986, 25, 4743). This direct relation together with the resolved hyperfine lines significantly simplified the simulation of single-crystal spectra in the three principal directions due to the reduction of free parameters and, thus, allowed us to define the magnetic g and A tensors of the molecule with a high degree of reliability. These parameters were subsequently used to generate the solution EPR spectra at both X- and Q-bands with excellent agreement. The anisotropic g and hyperfine tensors determined by the simulation of the X- and Q-band single-crystal and solution EPR spectra are as follows: g(x) = 1.9887, g(y) = 1.9957, g(z) = 1.9775, and hyperfine coupling constants are A(x)(III) = \171\ G, A(y)(III) = \176\ G, A(z)(III) = \129\ G, A(x)(IV) = \77\ G, A(y)(IV) = \74\ G, A(z)(IV) = \80\ G.