The combustion synthesis (CS) of materials is an advanced approach in powder metallurgy. Our previous studies of various CS systems have shown that at small time scales (10(-3)10(-4) s), the behavior of self-propagating high-temperature reaction waves in heterogeneous mixtures follows two modes, classified according to their microstructures: quasihomogeneous and scintillating reaction waves. The latter is a novel phenomenon where the reaction propagates in a form of short and high-temperature flashes, so-called scintillations, which randomly arise in vicinity of the combustion front and promote its local movement. Using Ti-Si mixture as an example, this work is aimed to find quantitative relationship between reactant particle size and microstructural characteristics of the reaction front, including scintillation size and its distribution, as well as lifetime. It was shown that, for coarse powders (> 40 mum), specific number of Ti particles approximately equals the number of hot spots observed during the combustion process. This means that essentially every Ti particle forms a hot spot. However, for finer Ti particles, average hot spot is formed by several (3-5) such particles. Since size of scintillation decreases to a lesser extent than Ti particle size, the scintillation regime does not vanish when using finer Ti powders. These results confirm the general assumption of the relay-race model that the limiting factor of combustion wave propagation is heat transfer between reaction cells rather than reaction within the cell itself. (C) 2004 Published by Elsevier Ltd.