Electrical and structural properties of annealed Al films on GaAs were evaluated for the purpose of identifying reactions at the Al–GaAs interface. After annealing at 400° or 500°C (1 h, in vacuo), Al films on GaAs are covered with an array of erosion centers that expose the substrate. The Schottky barrier height increases with annealing temperature from 0.73 eV, with the ideality parameter n?1.03 in as-prepared contacts, to 0.82 eV, with n? 1.07, for contacts annealed at 500°C (1 h, in vacuo). Data from both selected-area transmission electron diffraction (TED) and dark-field transmission electron microscopy reveal the presence of Al precipitates in the near surface region of the GaAs in samples annealed at 500°C (2 h, in vacuo). Selected-area TED patterns obtained from alloyed Al films on (310) -oriented GaAs indicate the formation of AlAs crystallites at the interface. Chemical depth profiles obtained with the use of Auger electron spectroscopy provide further indication of thermally induced interdiffusion in Al–GaAs structures.