We have identified a class of zero-dimensional classical and quantum Heisenberg spin systems exhibiting anomalous behavior in an external magnetic field B similar to that found for the geometrically frustrated Kagomé lattice of classical spins. Our calculations for the isotropic Heisenberg model show the emergence of a pronounced minimum in the differential susceptibility dM/dB at Bsat/3 as the temperature T is raised from 0 K for structures based on corner-sharing triangles, specifically the octahedron, cuboctahedron, and icosidodecahedron. We also provide the first experimental evidence for this behavior dM/dB: It is exhibited by the giant Keplerate magnetic molecule {Mo72Fe30} (Fe 3+ ions with spin s = 5/2 on the 30 vertices of an icosidodecahedron). The minimum in dM/dB is due to the fact that for low temperatures when B ≈ Bsat/3 there exist two competing families of spin configurations of which one behaves magnetically "stiff" leading to a reduction of the differential susceptibility. The magnetism of frustrated one-, two-, and threedimensional lattice spin systems is a fascinating subject due to the richness of phenomena that are observed [1,2,3]. In this Letter we report that one effect of geometrical frustration, which so far has been reported [4] only for the theoretical model of classical spins on a Kagomé lattice, already appears for a class of zero-dimensional materials, namely certain magnetic molecules hosting highly symmetric arrays of classical or quantum spins. These molecular units [5] contain a set of paramagnetic ions whose mutual interactions are described by isotropic Heisenberg exchange and where the intermolecular magnetic interactions (dipole-dipole for the most part) are negligible as compared to intramolecular Heisenberg exchange. Magnetic molecules as zero-dimensional spin systems provide a new avenue for detailed exploration of the basic issues of geometric frustration. They are particularly appealing since they offer the prospect of being modeled unencumbered by some of the complications of bulk magnetic materials.We here report experimental and theoretical results for the occurrence of a striking anomaly in the differential susceptibility dM/dB versus magnetic field B that is exhibited by the giant Keplerate magnetic molecule {Mo 72 Fe 30 } [6,7]. This molecule features 30 Fe 3+ ions on the vertices of an icosidodecahedron that interact via nearest-neighbor (nn) isotropic antiferromagnetic (AF) exchange (J/k B = 1.57 K). Due to their near-perfect O h -symmetric coordination environment, the Fe 3+ ions represent ideal s = 5/2 spin centers with virually no single-ion anisotropy. We also present theoretical results for the classical and quantum Heisenberg model showing that the same anomaly in dM/dB occurs for a class of geometrically frustrated zero-dimensional systems, where spins mounted on the vertices of a triangle, octahedron, cuboctahedron, or an icosidodecahedron interact via nn isotropic AF exchange. As the temperature T is raised from 0 K a deep narrow minimum in dM/dB emerg...