Charge transfer excitation at 600 nm of the ion pair [Co(Cp)(2)(+)Co(CO)(4)(-)] prepares a neutral pair that can undergo spontaneous electron transfer. Electron transfer rates were measured by picosecond transient IR absorption for quantum numbers of 0 and 1 in the CO stretching mode of the Co(C0)(4) species. The rate of electron transfer was 2 times faster with one quantum of energy in the CO stretch mode. The electron transfer returns the ion pair species, and we measured transient IR for levels having 2, 3, and 4 quanta in the ion pair. The rise time of these levels is consistent with the measured rates of leaving the neutral pair. A model calculation was done that shows approximate agreement with the observed ratio of electron transfer rates. This is the first observation of electron transfer rates with vibrational resolution.