One-pot reaction of an asymmetrical acylhydrazone ligand H3L with Ln(ClO4)(3) and CuCl2 in MeOH and MeCN solvents resulted in two novel octanuclear complexes, [Cu4Dy4L4Cl6(CH3OH)(8) (H2O)(4)]center dot Cl-2(CH3OH)(9)(H2O)(3) (1) and [Cu4Tb4L4Cl6(CH3OH)(2)(H2O)(10)]center dot Cl-2(H2O)(x) (2). Single-crystal X-ray diffraction studies revealed that these two complexes are isostructural and can be viewed as being built from two {Cu(2)Ln(2)L(2)} (Ln = Dy or Tb) butterfly-shaped units. Direct current magnetic susceptibilities and field-dependent magnetization measurements demonstrated the presence of strong ferromagnetic interaction between Cu-II and Ln(III) and magnetic anisotropy. Furthermore, alternating current (ac) magnetic measurements illustrated that these two complexes showed temperature- and frequency-dependent signals in the out-of-phase ac susceptibility under zero applied field, which are typical features of the slow relaxation of the magnetization for 1 and 2. The effective energy barrier (U-eff) for 1 was 54 K, which is one of the highest U-eff values yet reported for Cu-II/Ln(III) single-molecule magnets.