The structures and characterization are described of complexes that exhibit an out-of-phase AC magnetic susceptibility Chi(m)". The dodecanuclear complexes [Mn12O12(O2CR)(16)(H2O)(4)](z) contain 8Mn(III),4Mn(IV) or Mn-II,7Mn(III), 4Mn(IV) and possess ground state spins of S = 9 or 10 (z = 0) or S = 19/2 (z = -1). The distorted-cubane complexes [Mn4O3X(O2CR>(3)(dbm>(3)] contain 3Mn(III),Mn-IV and have been prepared with a variety of X groups; in every case, the ground state is S = 9/2. The anion [Mn11O10X6(O2CPh)(11)(H2O)(4)](2-) (9Mn(III), 2Mn(IV); X = NCO-, N-3(-)) contains two [Mn4O3X] distorted-cubanes linked by a [Mn3O4](+) unit; the ground state is S = 5. The cation [V4O2(O2CEt)(7)(bpy)(2)](+) (4V(III)) has a [V4O2](8+) butterfly core and a S = 3 ground state. All these complexes exhibit out-of-phase AC magnetic susceptibility signals at low temperatures (<8 K). These signals are indicative of slow relaxation of the magnetization, unable to keep in phase with the oscillating AC magnetic field, and are taken as evidence for single-molecule magnetism. The origin of the barrier to relaxation is discussed.