The Al3Ni formation reaction was examined at 11 heating rates ranging from 1000 K/s to 100,000 K/s using high-rate nanocalorimetry, time-resolved electron microscopy, isoconversional analysis, and combined kinetic analysis. Two main reaction steps are identified. In the first, interdiffusion occurs between reactants without product nucleation. The estimated activation energy for this process, 113 kJ/mol +/- 4 kJ/mol, suggests grain boundary diffusion as the most likely controlling mechanism. In the second step, the Al3Ni product phase nucleates and grows. Here, the estimated activation energy of 137 kJ/mol +/- 4 kJ/mol suggests that growth is enabled by diffusion of Ni through bulk Al. Combined kinetic analysis of the growth regime yields an Avrami reaction model with an exponent of 0.5, implying 1D diffusion-limited growth from a fixed number of randomly distributed nuclei. Combining the results for the two regimes, we propose a mechanism where the Al3Ni product initially nucleates along the grain boundaries and then grows laterally until the reactants are consumed.