A series of mixed-valent heterometallic pentanuclear Co^III₃Co^IIRE^III (RE = Gd (1), Tb (2), Y (3)) clusters have been rationally assembled by taking advantage of a bifunctional ligand with o-vanillin and tripodal tris(hydroxymethyl)aminomethane units. Structural determinations reveal that all compounds are isomorphous and possess a T-shaped Co₄RE core, which comprises two nearly linear Co₂RE subunits sharing a common RE ion. Their magnetic properties were thoroughly studied. The static magnetic susceptibility studies of 1 demonstrate the presence of weak ferromagnetic interactions between the magnetic centres and magnetic anisotropy reflected by the single ion zero-field splitting (ZFS) D term. Both 1 and 2 behave magnetically as heterodinuclear metal systems, while the magnetic behaviour of 3 is identical to an isolated Co^II ion. Moreover, alternating-current susceptibility measurements did not exhibit any out-of-phase signal, suggesting that slow magnetic relaxation is absent above 2 K within them. These isomorphous Co₄RE clusters offer an opportunity to systematically probe the contribution of different metal ions to the overall magnetic behaviour in Co^II–RE^III systems.