Interfacial reactions between iridium and gallium arsenide in both the thin‐film and bulk forms have been investigated in the temperature range 400 to 1000 °C using transmission electron microscopy, energy dispersive x‐ray analysis, and electron probe microanalysis. The diffusion path for Ir/GaAs has been determined to be Ir/IrGa/IrAs2/GaAs and is consistent with the phase diagram between the initial stages of reaction (thin‐film) and long‐term annealing (bulk). In the thin film case where the Ir supply is limited, the final configuration is Ir3Ga5/ IrAs2/GaAs. The diffusion path and reaction morphology has been rationalized using the phase diagram, kinetic observations, and growth mechanisms observed during the reactions.