We showed how intermetallic formation reactions can be studied under rapid heating (10$^6$–10$^7$ K/s) using x-ray microdiffraction with temporal resolution on microsecond time scales. Rapid heating was achieved by initiating an exothermic reaction in multilayer foils comprising alternating nanoscale layers of elemental metals. The reaction occurred in a front ∼100 $\mu$m wide which propagated across the foil at ∼1–10m/s. By using synchrotron x-rays focused to a small spot (60$\mu$m diameter) and a fast pixel-array detector, we were able to track the evolution of phases in the reaction front during the initial heating transient, which occurred in approximately 1 ms, through cooling over a period of hundreds of milliseconds. In Al/Ni multilayer foils, the first phases to form were an Al-rich liquid and the cubic intermetallic AlNi (which likely formed by nucleation from the liquid). In foils of overall composition AlNi, this is the stable intermetallic and the only phase to form. In foils of composition $Al_3Ni_2$, during cooling we observed a peritectic reaction between AlNi and the remaining liquid to form Al$_3$Ni$_2$, which is the stable phase at room temperature and the final product of the reaction. This is in contrast to the sequence of phases under slow heating, where we observed formation of nonequilibrium $Al_9Ni_2$ first and do not observe formation of a liquid phase or the AlNi intermetallic. We also observed formation of an amorphous phase (along with crystalline ZrNi) during rapid heating of Zr/Ni multilayers, but in this system the temperature of the reaction front never reached the lowest liquidus temperature on the Zr–Ni phase diagram. This implies that the amorphous phase we observed was not a liquid arising from melting of a crystalline phase. We suggest instead that a Zr-rich amorphous solid formed due to solid-state interdiffusion, which then transformed to a supercooled liquid when the temperature exceeded the glass transition temperature. Formation of the supercooled liquid presumably facilitated continued rapid intermixing, which may be necessary to sustain a self-propagating reaction front in this system.