The mechanism of amine sensitization of shocked nitromethane was investigated using time-resolved optical absorption spectroscopy in the visible. Neat nitromethane and mixtures of nitromethane with six different (primary, secondary, tertiary, and di-) amines were shocked to within 12-17 GPa peak pressure using step wave loading. Despite the small amine concentrations, profound differences were observed in the absorption spectra of neat and sensitized nitromethane. The changes in the absorption spectrum of reacting neat nitromethane consisted of irreversible broad-band (450-650 nm) loss of transmission through the sample after a short induction time. In contrast, a transient absorption peak at 525 nm developed in the spectra of the reacting mixtures. This feature did not depend on the particular amine used. We assign it to a transient intermediate formed in the shocked mixtures during the early stages of decomposition. On the basis of our analyses and the data available in the literature, we identify the intermediate as a radical anion of nitromethane, CH3NO2.-. The implications of this on the mechanism of sensitization are discussed. Several possible radical anion mechanisms are considered and evaluated. The base catalysis by amines is favored as the most plausible mechanism of sensitization. This mechanism is discussed in detail.