The redox behavior of the cobaltacarborane sandwich complexes CP*Co[(Me(3)Si)(2)C2B4H4] (1), Cp*Co[(Me(3)Si)C2B4H5] (2), {Cp*Co[Me(3)Si)C2B4H4]CH2}2C6H4 (4), Cp*Co(Et(2)C(2)B(4)H(4)) (5), and Cp*Co(Me(2)C(2)B(4)H(4)) (6) was investigated via cyclic voltammetry, proton NMR spectroscopy, and ESR spectra of paramagnetic anionic species. The reductions of the neutral diamagnetic monocobalt complexes 1 and 2 to their monoanions and of the dicobalt species 4 to the dianion were studied by recording in each case a series of NMR and ESR spectra during intermittent exposure to a potassium mirror until complete conversion to the anion was achieved. The NMR chemical shift correlations so obtained allow complete assignment of the paramagnetic NMR signals and afford information on the effects of reduction in different regions of the molecules. The NMR and ESR data indicate that reduction takes place primarily at cobalt. In general, as probed by H-1 NMR spectroscopy, the effects of reduction are strongest for the Cp* protons and are weaker for those further from the paramagnetic metal center. The methyl protons in SiMe(3) groups attached to the carborane are more strongly affected than are the methyl protons of carborane-bound ethyl groups. These findings are consistent with the ESR spectra of the paramagnetic anions, which show smaller cobalt hyperfine coupling constants for the silyl-substituted species than for the ethyl derivatives.