Dichromium oxide cluster anions, Cr2On- (n=1-3), were found to possess highly spin-polarized electronic structures, which were revealed by the measurements of photoelectron spectra and the analyses by the density-functional calculations. Their spin magnetic moments were as large as 9, 9, and 7 mu(B) for n=1, 2, and 3, respectively, due to a ferromagnetic coupling between local spins on the chromium atoms. The ferromagnetic spin couplings were caused predominantly by a superexchange-type Cr-Cr interaction through an oxygen atom at the bridge site, where a significant mixing of Cr 3d with O 2p orbitals stabilized the ferromagnetic states. The high-spin characters of Cr2On- are in striking contrast to that of a pure chromium dimer, which is known to exhibit an antiferromagnetic spin coupling due to the strong Cr-Cr covalent bond. The present ferromagnetic spin couplings should, therefore, be induced by oxidation. These findings support a concept that a chemical reaction controls magnetic properties of molecules and clusters. (C) 2003 American Institute of Physics.