The isostructural tridecanuclear mixed-metal [FeIII5LnIII8] (Ln = Pr, Nd, Gd) aggregates are the largest [Fe-Ln] clusters reported to date and the magnetic properties suggest a ferrimagnetic arrangement in [Fe5Pr8] and [Fe5Nd8], whereas for [Fe5Gd8] ferromagnetic interactions are dominant leading to a large spin ground state.