The extreme reaction conditions during combustion synthesis (CS) process require new methods for controlling the microstructure (and hence, the properties) of the desired materials. It appears that the structural changes which determine the microstructure and phases are influenced by local variations in reactant composition and temperature. A direct observation of the microstructural transformations during CS was made using a novel digital high-speed microscopic video recording technique, with imaging rates up to 12,000 frames/s and magnification up to 800 x. It was shown that over a wide range of experimental conditions, macroscopically steady reaction fronts in heterogeneous mixtures exhibit random microscopic fluctuations in shape and instantaneous velocity, which are directly related to the microstructure of the reaction mixture. These fluctuations were correlated with the initial heterogeneity of the reaction medium. In conjunction with these experiments, a micro-heterogeneous cell model was developed to describe the local reaction wave propagation in a heterogeneous porous medium. The calculated combustion front profiles compared well with the experimental results.