The synthesis, X-ray crystal structure and magnetic properties of a new single-molecule magnet [Mn8Fe4O12-(O 2CCH2Cl)16(H2O)4] ?2CH2ClCOOH?CH2Cl2 (3?2CH2-ClCOOH?2CH2Cl2) are reported. Reduced magnetization data of complex 3 indicate an S = 4 ground state, and the out-of-phase components of AC magnetic susceptibilities have frequency dependent maxima. Copyright ? 2005 The Chemical Society of Japan.