Chemical reactions at the Au/InP interface were investigated in the temperature range 25°-510 °C by x-ray diffractometry, scanning electron microscopy (SEM), and energy dispersive x-ray analysis. The samples were prepared by depositing gold films onto clean InP (100) single-crystal substrates under 10∼9 Torr vacuum. Spots appeared in SEM micrographs of the deposited film of a sample annealed at room temperature for 35 days, indicating that a solid-state reaction had occurred. After the sample was heated to 330 °C under flowing N2 gas, £1 dendritic islands were observed in the film. A 365 °C anneal turned the color of the Au film from yellow to pink, and Au, £1 and Au2P3 were identified by x-ray diffractometry. In reacting with the InP substrate, £1 produced silver-colored γ and more Au2P3 at 450 ° C. With respect to InP, γ was relatively stable, and further reactions were minimal even when the sample was annealed at 510 °C for 40 min under atmospheric-pressure N2. The ternary phase diagram for the bulk Au-In-P system provides the basis for understanding the sequence of the above results and much of the information in the literature about Au/InP interfacial reactions.