The reaction of manganese(II) acetate, 1,1,1-tris(hydroxymethyl)-methane (H(3)thme), and triethylamine in methanol leads to the formation of [Mn12O2(OMe)(2)(thme)(4)(OAc)(10)(H2O)(4)]center dot 2MeOH. The [(Mn4Mn8II)-Mn-III] core consists of a central [(Mn4O6)-O-III] rhombus sandwiched by two [(Mn4O7)-O-II] fragments. Frequency-dependent ac susceptibility and hysteresis loops in the magnetization indicate single-molecule magnet behavior with a pure quantum-tunneling regime of relaxation below 0.2 K.