Electronic and chemical changes in shock-compressed nitromethane and a mixture of nitromethane with ethylenediamine (0.1 wt %) have been examined using time-resolved optical absorption spectroscopy under stepwise loading to 14 GPa. Despite the small amine concentrations, profound differences are observed in the absorption spectra of pure and sensitized nitromethane. The changes in the absorption spectrum of pure nitromethane involve a maximum absorption edge shift of 20 nm and are completely reversible. In contrast, the mixture shows an irreversible time-dependent shift in the absorption edge of up to 90 nm toward longer wavelengths. A substantial component of the shift takes place after the sample has reached peak pressure, The time dependence and irreversibility of the absorption edge shift are caused by a chemical change. They confirm the sensitization of nitromethane in the presence of amines. The observed shift in the absorption edge is treated as a kinetic measurement. The derived kinetic parameters of the system are consistent with a previously suggested chemical mechanism for the sensitization of nitromethane by amines.