A new family of Co/rare-earth intermetallic cubane aggregates [Co(3)Ln(hmp)(4)(OAc)(5)H2O] (Ln = Dy, Ho, Er, Tm, Yb, Y) have been synthesized by self-assembly. Single-crystal X-ray diffraction analysis revealed that they are remarkably isostructural in showing a common [Co(3)Ln] core. Magnetic studies showed that the Dy, Er, Tm, Yb, and Y complexes are ferromagnetic. The Dy complex exhibits the largest magnetocaloric effect (-Delta S-m = 12.58 J kg(-1) K-1), which can be attributed to the large magnetic density of Dy-III.