Time-resolved UV-visible absorption spectroscopy was used to examine chemical decomposition in neat liquid nitromethane (NM) subjected to stepwise shock loading to 19 GPa. Up to a peak pressure (temperature) of 13.8 GPa (854 K), no sign of chemical reaction was observed in the n pi* absorption band centered at 270 nm. For shock compression resulting in peak temperatures above 940 K, extensive reaction was indicated by irreversible red-shifting of the absorption band edge that occurred after peak pressure was reached. This red-shift was followed by a loss of transmission through the sample, which was attributed to the formation of absorbing reaction products. Comparison of these reaction-induced spectral changes with previous absorption results for NM sensitized by ethylenediamine (EDA) suggests that the presence of the amine causes a change in the early stages of shock-induced decomposition. An induction time was observed between the attainment of peak pressure and the onset of reaction-induced spectral changes in neat NM. Significant decreases in the induction time were produced by modest increases of 25-50 K in the initial sample temperature. The induction time data are consistent with the thermal explosion model of shock initiation in energetic materials. The observed induction time correlates well with the final shock temperature; no pressure dependence is observable within the pressure range examined here. Measured induction times for the absorption experiments are consistently shorter than for continuum experiments reaching similar temperatures likely because the absorption technique probes earlier stages of the reaction process. This suggests that induction times measured using different experimental techniques are not necessarily equivalent.