An isostructural family of tetranuclear aggregates [Mn(2)(III)Ln(2)(O)(Piv)(2)(heP)(4)(NO3)(4)]center dot MeCN (where Ln = Y-III (1) Pr-III (2), Nd-III (3), Gd-III (4), Tb-III (5), Dy-III (b) Ho-III (7), and Yb-III (8)) is reported. They were obtained from the reactions of 2-(2-hydroxyethyl)pyridine (hepH) with a preformed hexanuclear manganese complex, [Mn-6], and the respective lanthanide salt. The complexes are isomorphous and represent a new heterometallic 3d-4f complex type for this class of ligand. The structural core of 1-8 consists of a distorted Mn(2)Ln(2) tetrahedron with the four metal centers linking through a mu(4)-O2- bridging atom. The magnetic properties of all complexes were investigated by variable temperature magnetic susceptibility and magnetization measurements. The magnetic data of all compounds suggest that antiferromagnetic interactions are present between adjacent paramagnetic centers. Complexes 5-7 containing highly anisotropic lanthanide ions (Tb, Dy, and Ho) show slow relaxation of their magnetization.