An analysis of the reaction of Au and Au‐In alloys with InP has permitted the identification of the mechanisms occurring during the first two stages of the Au‐InP interaction. The first stage of the interaction, during which the Au is converted to a saturated Au(In) solution, is controlled by the vacancy‐generation rate at the free surface of the metallization. The activation energy for this process is the activation energy for Au self‐diffusion. Evidence is presented for the existence of large localized variations in this value due to surface related effects. At the completion of stage I stage II becomes active and continues until the metallization is converted to Au3In. This process, proceeding via an interstitial interchange mechanism, is many orders of magnitude slower than stage I. The rate‐limiting step, with an activation energy of 2.8 eV, is shown to be the diffusion of In from the InP‐metal interface. The P atoms that are released when In enters the metallization during stage I leave the system without reacting, whereas in stage II they form a compound (Au2P3) at the InP‐metal interface. The presence of the Au2P3severely retards the stage II interaction rate.