Propagating reactive waves through porous silicon (PS)-sodium perchlorate composites and the generation of shock waves in the gaseous medium above the PS surface were studied using high-speed shadowgraphy. Propagation speeds were varied by changing the PS specific surface area (SSA) and the dopant type and level, and by the addition of organized microstructures along the wave propagation direction. Shadowgraph analysis showed that upstream permeation of hot gaseous combustion products was responsible for a two order of magnitude enhancement in the reactive wave propagation speeds obtained by the presence of organized microscale patterns on PS samples with low SSA (similar to 300 m(2)/g), which nominally exhibit baseline speeds of similar to 1 m/s. Shadowgraph analysis and sound speed measurements on PS samples with high SSA (similar to 700 m(2)/g), which exhibit fast reactive wave propagations of similar to 1000 m/s, indicated that neither the strong shock over the PS surface nor detonation of the porous layer were the mechanisms by which the wave propagated. Thermal analysis of PS showed that the heat release from exothermic reactions between PS and the oxidizer within the pores shifted to lower temperatures as the SSA of PS increased, which was accompanied by a reduction in the activation energy associated with the lowest temperature exothermic reaction between PS and the oxidizer. The combined experiments indicated that a combination of conductive and convective burning, possibly assisted by fast crack propagation within the silicon/porous silicon substrate, was responsible for the observed difference in propagation speeds and was the mechanism by which the reactive wave propagated with speeds on the order of a km/s within the porous layers.