The reaction of 2,2’-biphenoxide with simple manganese halide salts affords novel mixed-valent decanuclear complexes of formula [Mn10O4(biphen)(4)X(12)](4-) (X = Cl-, Br-). The syntheses of three related compounds in this class, [Me(4)N](4)[Mn10O4(biphen)(4)Cl-12] (1), [Me(4)N]4[Mn10O4(biphen)(4)Br-12] (2), and [Mn(CH3CN)(4)(H2O)(2)][Et(3)NH](2)[Mn10O4(biphen)(4)Br-12] (3), are described. These complexes have : been structurally characterized by X-ray diffraction, details of which are reported for 2. Compounds 1-3 belong to a very small class of high-nuclearity manganese clusters held together by bridging ligands other than the carboxylate moiety. They are mixed-valent, with four Mn(III) atoms and six Mn(II) atoms comprising the cluster. All of the manganese centers are valence-localized, the octahedral Mn(III) sites being delineated by marked Jahn-Teller axial elongations. The bridging ligands bind in a variety of modes, there being four mu(4)-oxo ligands at the core of the cluster. The magnetic properties of compounds 1-3 were investigated in the polycrystalline state by magnetic susceptibility and high-field magnetization measurements, which revealed high-spin ground states (12 less than or equal to S less than or equal to 14). The chi T curve for 2 steadily increased from 37.9 emu mol(-1) K at 300 K to 95.1 emu mol(-1) K at 5 K. The high-field magnetization curve reached a plateau at a value of 28.3 mu(B) for the reduced magnetization. Single-crystal susceptibility and X-band cw-EPR measurements on 3 showed an Ising-type anisotropy in the magnetization for this compound, with chi(parallel to)T = 128 emu mol(-1) K at 2.2 K, and chi(perpendicular to) T = 46 emu mol(-1) K at 2.1 K. The magnitude of the overall zero-field splitting parameter, D, for the S = 12 ground state was calculated to be D = 0.035 cm(-1) from the single crystal EPR data. Theoretical calculations based on simplified models are presented which provide a framework in which to understand the origin of the high-spin ground states for these complexes.