Metastable solid phases can be produced by rapid quenching from the liquid state, which limits the time available for diffusion and nucleation and growth of competing phases. Similar effects can be expected due to rapid heating, but most studies of phase transformations involving interdiffusion of chemical species have been performed either isothermally or at moderate heating rates (~1 K/s). In this talk, we discuss the influence of rapid heating (~10^6 K/s) on phase formation during self-propagating reactions in metallic multilayers. The reaction front is only ~100 µm wide and moves at ~1 m/s, but using time resolved synchrotron x-ray microdiffraction we can follow the sequence of phase formation in detail. For example, we show that the intermediate phases formed during self-propagating reactions in Al/Ni multilayers are different from those formed at lower heating rates, even though the final phases are the same. In situ characterization is essential, as other means of studying self-propagating reactions (such as quenching the reaction followed by ex situ analysis) provide different - and potentially misleading - results. We will interpret our observations in light of the effect of heating rate on the competitive nucleation and growth of the various intermetallic phases.