Ternary phases of the type M XA IIIB V prepared by solid-phase reaction between a metal, M, and a substrate, A IIIB V, are potentially useful materials for forming shallow and adherent contacts to III–V semiconductors. In this letter, the results of a transmission electron microscopy study of the Pd-GaAs reaction are presented. It is shown that the first two reaction products are ternary phases of the type M XA IIIB V (Pd 5(GaAs) 2 and Pd 4GaAs). By choosing appropriate metal thicknesses and annealing temperatures, uniform monocrystalline films of Pd 5(GaAs) 2, free of interfacial accumulations of As and Ga, can be obtained. As contacts to GaAs, these Pd XGaAs films are analogous to suicide contacts to silicon.