A flashover arc source that delivered up to 200 mJ on the 100s-of-ns time-scale to the arc and a user-selected dielectric surface was characterized for studying high-explosive kinetics under plasma conditions. The flashover was driven over thin pentaerythritol tetranitrate (PETN) and poly(methyl methacrylate) (PMMA) dielectric films and the resultant plasma was characterized in detail. Time- and space-resolved temperatures and electron densities of the plasma were obtained using atomic emission spectroscopy. The hydrodynamics of the plasma was captured through fast, visible imaging. Fourier transform infrared spectroscopy (FTIR) was used to characterize the films pre- and post-shot for any chemical alterations. Time-resolved infrared spectroscopy (TRIR) provided PETN depletion data during the plasma discharge. For both types of films, temperatures of 1.6–1.7 eV and electron densities of ∼7–8 × 1017/cm3 ∼570 ns after the start of the discharge were observed with temperatures of 0.6–0.7 eV persisting out to 15 μs. At 1.2 μs, spatial characterization showed flat temperature and density profiles of 1.1–1.3 eV and 2–2.8 × 1017/cm3 for PETN and PMMA films, respectively. Images of the plasma showed an expanding hot kernel starting from radii of ∼0.2 mm at ∼50 ns and reaching ∼1.1 mm at ∼600 ns. The thin films ablated or reacted several hundred nm of material in response to the discharge. First TRIR data showing the in situ reaction or depletion of PETN in response to the flashover arc were successfully obtained, and a 2-μs, 1/e decay constant was measured. Preliminary 1 D simulations compared reasonably well with the experimentally determined plasma radii and temperatures. These results complete the first steps to resolving arc-driven PETN reaction pathways and their associated kinetic rates using in situ spectroscopy techniques.