The reactions between (100) GaAs and the near-noble metals Ni, Pd, and Pt have been investigated by application of high-resolution transmission electron microscopy (TEM), energy-dispersive analysis of x-rays in the scanning TEM and Rutherford backscattering spectrometry. Emphasis is placed on the evolution of the phase distributions, film compositions, and interface morphologies during annealing at temperatures up to 480°C. The first phase in the Ni/GaAs reaction is shown to have the nominal composition Ni3GaAs. Ternary phases of the type PdxGaAs are also found to be the dominant products of the Pd/GaAs reaction. Conversely, only binary phases result from the Pt/GaAs reaction. These observations are used to construct isothermal sections of the M-Ga-As thin-film phase diagrams. The behavior of a thin (1–2 nm) native oxide-hydrocarbon layer during the Ni/GaAs, Pd/GaAs, and Pt/GaAs reactions is also investigated. Only the Ni/GaAs reaction is noticeably impeded in some regions by this intervening layer. In contrast, the Pd/GaAs and Pt/GaAs reactions tend to mechanically disperse the native oxide layers.