For many condensed heterogeneous systems. thr propagation of the combustion front occurs in a planar. steady manner, when observed using typical imaging rates (30 frames/s) and magnifications (Ix). However, using much higher magnification (> 400x) and imaging rates (1000 frames/s). two different mechanisms (quasiho-mogeneous and relay-race) oi combustion propagation have been observed recently. For the former, the microstructure of the combustion wave resembles what is viewed macroscopically (i.e., at typical magnification and imaging rates). Under these conditions. steady, planar propagation occurs. In the relay-race mechanism, while planar at the macroscopic level, the combustion front profiles are irregularly shaped, with are-shaped convexities and concavities at the microscopic level. Moreover. the reaction front propagation consists of a series of rapid jumps and hesitations. In this paper, based on the local conditions during combustion (i.e., the microstructure of the combustion wave), we develop and evaluate several new criteria for determining the mechanisms of combustion wave propagation. Using these criteria, the boundaries between quasihomogeneous and relay-race mechanisms are determined experimentally as a function of the initial organization of the reaction medium (i.e., particle size and porosity). Further, the characteristics of the two mechanisms in terms of temperature gradients and ignition delays are also studied.