Electron transfer involves changes in molecular geometry that are important for controlling rates. In this work we report the first clear effects of vibrational quantum state on solution phase electron transfer rates. The spontaneous electron transfer rates for the recovery of an ion pair [Co(Cp)(2)(+)/V(CO)(6)(-)] are studied with picosecond infrared spectroscopy following optical excitation into its charge transfer band. The rates increase about 2-fold for each additional vibrational quantum in the CO stretching mode. These results allow new tests of electron transfer theory.