A high-speed photographic study has been made of the initiation mechanisms during low-velocity mechanical impact of high explosives in the presence of solid glass beads and hollow glass and polymeric microspheres. In addition, a few experiments were carried out with aluminium and polycarbonate (PC) particles as additives. The explosive chosen was an oxygen-balanced mixture of triaminotrinitrobenzene (TATB) with either ammonium perchlorate (AP) or ammonium nitrate (AN). While the explosive mixture was found to be insensitive to impact, all the additives except aluminium and PC had a sensitizing effect, although by different amounts. The photographic sequences revealed that initiation and propagation events are much more rapid with the glass beads compared to those with microspheres. It is believed that the “hot spots”, leading to faster decomposition rates, are generated via frictional or compressive heating mechanisms depending upon the nature of the inhomogeneity in the TATB/oxidizer/additive mixture. The results have implications for the safe handling of explosives.