The objective of this work is to improve the knowledge of the shock-to-detonation transition of nitromethane. The study is based on a spectral analysis in the range 0.3–0.85 µm, with a 28-nm resolution, during experiments of plane shock impacts on explosive targets at 8.6 GPa. The time-resolved radiant spectra show that the detonation front, the reaction products produced during the superdetonation, and the detonation products are semitransparent. The temperature and absorption coefficient profiles are determined from the measured spectra by a mathematical inversion method based on the equation of radiative transfer with Rayleigh scattering regime. Shocked nitromethane reaches at least 2500 K, showing the existence of local chemical reactions after shock entrance. Levels of temperature of superdetonation and steady-state detonation are also determined.