The results from a multifrequency high-field EPR study of five di-mu-oxo bridged mixed-valence binuclear Mn(III)Mn(IV) complexes are reported. Spectra were obtained at 9, 95, and 285 GHz. The g anisotropy was unambiguously observable at 285 GHz. Hyperfine and g tensor values were estimated using spectral simulation procedures that cyclically and simultaneously fit the multifrequency data. In all five cases, the g tensors of the mixed-valence complexes were found to be rhombic. The g tensors were analyzed using the vector projection model. Most, but not all, of the g anisotropy originates from the Mn(III) center. The rhombic g tensors result from the low symmetry of the manganese centers. The size of the effective g anisotropy for a given complex was found to be a linear function of the average bond distance between the manganese and axial nitrogens. This relationship can be understood in terms of the influence of tetragonal distortion on the electronic levels of the Mn(III) center. The frequency-dependent line broadening observed in these mixed-valence complexes is explained in terms of the relationship between g anisotropy and structure.