The reaction of 6-formyl-2-(hydroxymethyl)-4-methylphenol (LH2) with appropriate lanthanide salts followed by reaction with Co(OAc)2·4H2O afforded the pentanuclear heterobimetalllic compounds [Co4Ln(L)4(OAc)2(S)4](NO3)(S) [LnIII = GdIII, S = MeOH (1); LnIII = DyIII, S = H2O (2); LnIII = TbIII, S = MeOH (3); LnIII = HoIII, S = MeOH (4)] in good yields. All the compounds are stable in solution as confirmed by ESI-MS studies. These complexes contain a distorted Co4 tetrahedral core, which encapsulates a central lanthanide ion. The CoII and LnIII ions are in an all-oxygen environments. All the CoII ions possess a distorted octahedral geometry, and the LnIII ions are in a distorted square-antiprismatic geometry. The magnetic properties of 1–4 have been investigated in the temperature range 2–300 K. In addition, the magnetic properties of 1 were fitted by using a Hamiltonian of the type H = HSO + Haxial + Hexchange + HZeeman. A moderately strong antiferromagnetic interaction between the nearest-neighbour CoII ions was found, whereas the magnetic interaction between the CoII ions and the corresponding LnIII ions is negligible.