The synthesis and magnetic properties of three isostructural hexadecametallic manganese clusters [Mn16O16(OMe)(6)(O2CCH2Ph)(16)(MeOH)(6)] (1), [Mn16O16(OMe)(6)(O2CCH2Cl)(16)(MeOH)(6)] (2), and [Mn16O16(OMe)(6)(O2CCH2Br)(16)(MeOH)(6)] (3) are reported. The complexes were prepared by a reductive aggregation reaction involving phenylacetic acid, chloroacetic acid or bromoacetic acid, and (NBu4MnO4)-Mn-n in MeOH. Complex 1 crystallizes in the monoclinic space group C2/c and consists of 6 Mn-IV and 10 Mn-III ions held together by 14 mu3-O-2-, 2 mu-O-2-, 4 mu-MeO-, and 2 mu-O2CCH2Ph- groups. The remaining 14 mu-O2CCH2Ph- ligands, 2 mu-MeO- groups, and 6 terminal MeOH molecules constitute the peripheral ligation in the complex. Variable-temperature, solid-state dc magnetic susceptibility measurements on 1-3 in the temperature range 5.0-300 K reveal that all three complexes are dominated by intramolecular antiferromagnetic exchange interactions. Low-lying excited states preclude an exact determination of the spin ground state for 1-3 by magnetization measurements. Alternating current susceptibility measurements at zero dc field in the temperature range 1.8-10 K and a 3.5 G ac field oscillating at frequencies in the 5-1488 Hz range display, at temperatures below 3 K, a nonzero, frequency-dependent chi(M)" signal for complexes 1-3, with the peak maxima lying at temperatures less than 1.8 K. Single-crystal magnetization versus dc field scans down to 0.04 K for complex 1 show hysteresis behavior at <1 K, establishing 1 as a new member of the SMM family. No clear steps characteristic of quantum tunneling of magnetization (QTM) were observed in the hysteresis loops.