Nanothermite reactions are mechanistically not well understood, due to their ultra-fast transient nature, and the complexity of probing both the vapor-phase and condensed-state chemistries. In this work we examine the combustion product particles of three nano-sized thermite systems (Al/CuO, Al/WO3, Al/Bi2O3) as a probe of the underlying mechanism. Electron Microscopy (EM) and Energy-dispersive X-ray Spectroscopy (EDX) were used to evaluate the combustion product particle size distribution and composition. The results show two distinct product particle size distributions common to all three oxidizers. The larger particles are super-micron (though the precursors were nano-sized) and comprise approximately 90% of the product mass. Simple scaling arguments show that the large population cannot be formed from the vapor given the available residence time. The smaller distribution is sub-100 nm which is primarily the reduced metal formed from vapor phase condensation. This result implies that the majority of the global reaction and thus the energy release is occurring in the condensed phase. Based on these results, a phenomenological mechanism for the nanoaluminum based thermite reaction is proposed. (C) 2014 Published by Elsevier Inc.