The following dinuclear exchange-coupled manganese complexes are investigated: [dtneMn(III)Mn(IV)-(mu O-)(2)mu-OAc](BPh4)(2) (dtne 1,2-bis(1,4,7-triazacyclonon-1-yl)ethane), [(CH3)(4)dtneMn(III)Mn(IV)(mu-O)(2)mu-OAc]-(BPh4)(2) ((CH3)(4)dtne 1,2-bis(4,7-dimethyl-1,4,7-triazacyclonon-1-yl)ethane), [(CH3)(4)dtneMn(III)Mn(IV)(mu-O)(2)mu-OAc](ClO4)(3), [(tacn)(2)(MnMnIV)-Mn-III(mu-O)(2)mu-OAc](BPh4)(2) (tacn = 1,4,7-triazasyclononane), [bpy(4)Mn(III)Mn(IV)(mu-O)(2)](ClO4)(3) (bpy = 2,2＇-bipyridyl), and [phen(4)Mn(III)Mn(IV)(mu-O)(2)](ClO4)(3) (phen 1,10-phenanthroline). For three of these complexes, X-ray structural data obtained on single crystals are reported here. All complexes are strongly antiferromagnetically coupled, with exchange coupling constants ranging from J = -110 cm(-1) (bis-mu-oxo-mu-acetato-bridged) to -150 cm(-1) (bis-mu-oxo-bridged). EPR investigations at X- and Q-band frequencies are reported for all five mixed-valence (MnMnIV)-Mn-III complexes. G tensors and Mn-55 hyperfine coupling constants (hfc＇s) were obtained by simultaneous simulation of the EPR spectra at both frequency bands. By using the vector model of exchange-coupled systems, tensor axes could be related to the molecular structure of the complexes. Hyperfine coupling constants from Mn-55 cw-electron-nuclear double-resonance (ENDOR) spectra were in agreement with those obtained from the simulation of the EPR spectra. Ligand hyperfine couplings (H-1 and N-14) were also measured using cw-ENDOR spectroscopy. Electron spin-echo envelope modulation spectroscopy (ESEEM) spectra yielded information about small N-14 hyperfine and quadrupole coupling constants that could not be resolved in the ENDOR spectra. On the basis of specifically deuterated complexes and results from orientation-selection ENDOR spectra, some proton hfc＇s could be assigned to positions within the complexes. Using an extended point-dipole model and the coordinates provided by the X-ray structure analysis, all dipolar hfc＇s of the complexes were calculated. Comparison of these hfc＇s with experimentally obtained values led to a consistent assignment of most hf tensors to molecular positions. The electronic structures of the investigated complexes are compared with each other, and the relevance of the results for metalloenzymes containing at least a dinuclear manganese core is discussed.