The syntheses, structures, and magnetic properties are reported of the mixed-metal complexes [Ce4Mn10O10(OMe)(6)(O2CPh)(16)(NO3)(2)(MeOH)(2)(H2O)(2)] (1) and [Th6Mn10O22(OH)(2)(O2CPh)(16)-(NO3)(2)(H2O)(8)] (2), which were both prepared by the reaction of (NBu4n)[Mn4O2(O2CPh)(9)(H2O)] (3) with a source of the heterometal in MeCN/MeOH. Complexes 1 and 2 crystallize in the monoclinic space group C2/c and the triclinic space group P (1) over bar, respectively. Complex 1 consists of 10 Mn-III, 2 Ce-III, and 2 Ce-IV atoms and possesses a very unusual tubular [Ce4Mn10O10(OMe)(6)](18+) core. Complex 2 consists of 10 Mn-IV and 6 Th-IV atoms and possesses a [Th6Mn10O22(OH)(2)](18+) core with the metal atoms arranged in layers with a 2:3:6:3:2 pattern. Peripheral ligation around the cores is provided by 16 bridging benzoates, 2 chelating nitrates, and either (i) 2 each of terminal H2O and MeOH groups in 1 or (ii) 8 terminal H2O groups in 2. Complex 1 is the largest mixed-metal Ce/Mn cluster and the first 3d/4f cluster with mixed-valency in its lanthanide component, while complex 2 is the first Th/Mn cluster and the largest mixed transition metal/actinide cluster to date. Solid-state dc and ac magnetic susceptibility measurements on 1 and 2 establish that they possess S = 4 and 3 ground states, respectively. Ac susceptibility studies on 1 revealed nonzero frequency-dependent out-of-phase (chi(M)' ') signals at temperatures below 3 K; complex 2 displays no chi(M)' ' signals. However, single-crystal magnetization vs dc field scans at variable temperatures and variable sweep-rates down to 0.04 K on 1 revealed no noticeable hysteresis loops, except very minor ones at 0.04 K assignable to weak intermolecular interactions propagated by hydrogen bonds involving Ce-III-bound ligands. Complex 1 is thus concluded not to be a single-molecule magnet (SMM), and the combined results thus represent a caveat against taking such ac signals as sufficient proof of a SMM.