The ability to switch a molecule between different magnetic states is of considerable importance for the development of new molecular electronic devices. Desirable properties for such applications include a large-spin ground state with an electronic structure that can be controlled via external stimuli. Fe-42 is a cyanide-bridged stellated cuboctahedron of mixed-valence Fe ions that exhibits an extraordinarily large S = 45 spin ground state. We have found that the spin ground state of Fe(42 )can be altered by controlling the humidity and temperature. Dehydration results in a 15 mu B reduction of the saturation magnetization that can be partially recovered upon rehydration. The complementary use of UV-vis, IR, L-2,L-3-edge X-ray absorption spectroscopy and X-ray magnetic circular dichroism is applied to uncover the mechanism for the observed dynamic behavior. It is identified that dehydration is concurrent with metal-to-metal electron transfer between Fe pairs via a cyanide pi hybridization. Upon dehydration, electron transfer occurs from low-spin {Fe-II(Tp)(CN)(3)} sites to high-spin Fe-III centers. The observed reduction in magnetization upon dehydration of Fe-42 is inconsistent with a ferrimagnetic ground state and is proposed to originate from a change in zero-field splitting at electron-reduced high-spin sites.