Two new Mn-16 clusters are reported: [Mn16O10(OH)(3)(OMe)(8)(O2CPhBut)(17)(MeOH)(5)] (2) and [Mn16O16(OMe)(6)(O2CPh)(12)(NO3)(4)(MeOH)(2)(H2O)(4)] (3). The complexes were obtained by reductive aggregation of MnO4- in CH2Cl2/MeOH, and oxidation of Mn-II and preformed ((NBu4)-Bu-n)[(Mn4O2)-O-III(O2CPh)(9)(H2O)] with Ce-IV, respectively. The core of 2 has a Mn-16(III) core with an unusual 1:2:3:4:3:2:1 layer structure and a W-shaped pleated topology, whereas 3 contains a central 2 X 3 Mn-6(IV) planar grid held within a nonplanar Mn-10(III) loop and is a rare example of a complex with nitrate ions bridging like carboxylate ions. Variabletemperature, solid-state dc susceptibility, and ac susceptibility studies reveal that 2 and 3 possess S = 12 and S = 8 ground states, respectively. Fits of dc magnetization data collected over a temperature range of 1.8-4.0 K and a magnetization range of 0.1-4 T were fit to give S = 12, D = 0.16(2) cm(-1), g = 1.98(3) for 2 and S = 8, D = -0.22(1) cm(-1), g = 1.99(2) for 3, where D is the axial zero-field splitting parameter. The ac in-phase (chi(M)'T) susceptibility below 15 K confirmed the ground-state spin values of 2 and 3, as determined from dc data, and the appearance of frequency-dependent out-of-phase (chi(M)'') signals revealed that both complexes are new single-molecule magnets (SMMs). Fits of the ac data gave U-eff = 49.7(1) K and tau(0) = 4.32 X 10(-9) s for 2 and U-eff approximate to 14.0 +/- 2 cm(-1) and tau(0) approximate to 3.2 +/- 0.5 X 10(-8) s for 3, where U-eff if is the effective barrier to magnetization relaxation and tau(0) is the pre-exponential factor. Thus, complexes 2 and 3 are two new members of a growing family of Mn-16 clusters, and two new examples of high-nudearity SMMs, with the U-eff for 2 approaching the value for the prototypical SMM family, [Mn12O12(O2CR)(16)(H2O)4].