The thermal stability of vacuum deposited thin metallic layers (namely Ta, Cr, Cr–Au, Mo, Mo–Au and W) on GaAs for short (1 h) and long (∠150 h) thermal anneals at temperatures up to 550°C have been studied using Rutherford backscattering of helium ions, scanning electron microscopy with energy and wavelength dispersive x-ray analysis, and secondary ion mass spectrometry. We attempt to generalize the nature of thermal reactions of the metals with GaAs and the competing oxidation reactions in a humid/oxidizing atmosphere in terms of simple physiochemical considerations.