The syntheses, structural determinations and magnetic studies of tetranuclear CoII–LnIII complexes (Ln = Y, Gd, Tb) involving orthovanillin as main ligand are described. The structural studies demonstrate that centrosymmetric tetranuclear Co2–Ln2 complex molecules with a defect-dicubane central core are obtained, with hexacoordinate Co ions in deformed octahedral environments and nine-coordinate lanthanide ions. The Co ions are linked by two hydroxo bridges and each Co ion is also involved in a double phenoxo-hydroxo bridge with the two Ln ions, so that each hydroxo group is triply linked to the two Co and one Gd ions. The four metal ions are coplanar. A ferromagnetic Co–Ln interaction operates in the Co2–Ln2 complexes (Ln = Gd, Tb), along with a D zero-field splitting term for the cobalt ion and a weak ferromagnetic Co–Co interaction. The SMM behaviour of the Co2–Gd2 complex is confirmed by observation of hysteresis loops, as a consequence of the slowing down of relaxation for this tetranuclear complex. The Co2–Tb2 complex does not behave as a SMM, what could result from a subtractive combination of the Tb and Co anisotropies and an increased transverse anisotropy, leading to large tunnel splittings and quantum tunneling of magnetization.