The synthesis and characterisation of three new mixed-valent manganese clusters [Mn(II)₄Mn(III)₁₆O₁₂(OH)₄(tea)₈(chp)₄]·6MeOH·4H₂O (1), [Mn(II)₆Mn(III)₄(teaH)₄(teaH₂)₂(tpaa)₆(F)₈]·2Et₂O·4MeCN (2) and [Mn(II)₆Mn(III)₄(teaH)₄(teaH₂)₂(2-bpca)₆(F)₈]·4MeCN (3) are reported. They were obtained by the reaction of simple manganese salts with triethanolamine (teaH₃), triethylamine (NEt₃) and the appropriate co-ligand. In the case of 1, 6-chloro-2-hydroxypyridine (Hchp) was used, for 2, triphenylacetic acid (tpaa) and 3, 2-biphenylcarboxylic acid (2-bpca). The core of 1 is a Mn₂₀ supertetrahedron, while the cores of 2 and 3 are identical and have distorted ring-like topologies. Variable-temperature, solid-state DC and AC magnetic studies were performed on 1-3 in the 2-300 K (DC) and 2-18 K (AC) ranges. Cluster 1 has a S = 9 ground state with excited S states, larger in value than 9, close in energy. No SMM features were apparent in 1. In contrast, clusters 2 and 3, with S = 12 or 13 ground states, and with excited S levels of lower value than 12 lying close in energy, do show SMM features, albeit below 2 K in their AC out-of-phase, frequency dependent data.