The decomposition mechanism in shocked pentaerythritol tetranitrate (PETN) was examined using time-resolved emission spectroscopy. PETN single crystals were subjected to stepwise loading along [100] and [110] to peak stresses between 2 and 13 GPa. Due to concurrent changes in the optical transmission of PETN, emission spectra were analyzed using the absorption data acquired separately under the same loading conditions. Analyses of the corrected emission data revealed two bands in the spectra at similar to3.0 and similar to2.4 eV. Both bands were observed in every experiment regardless of stress or crystal orientation. However, their relative and absolute intensities, and temporal behavior revealed stress and orientation dependence. The emission was identified as chemiluminescence from the nitronium ion, NO2+, on the basis of its electronic structure and properties. NO2+ electronic structure was analyzed using ab initio calculations, which showed transition energies matching those of the emitting intermediate observed experimentally. Several chemical pathways compatible with the formation of NO2+ are considered and evaluated. Finally, a four-step chemical initiation mechanism in shocked crystalline PETN is proposed and discussed in detail.