Electrical conductivity profiles were measured in three detonating high explosives with initial densities slightly greater than loose-packed ones. Behind the detonation fronts, the conductivity peaks were found presumably correlating with the reaction zones. A distinct grain size effect was demonstrated: the conductivity peaks were clearly shorter in fine-grained materials. This suggests that the chemical reaction rate is faster for smaller grains. This supports the concept of a "hot spots" reaction mechanism prevailing at comparatively low pressures. (C) 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.