CoSi2exhibits the features of low resistivity and stability at elevated temperatures which make it interesting to employ for metallization on GaAs. The interfacial reactions in GaAs samples with thin film overlayers of Si and Co [Si(220 nm)/Co(50 nm)/(⟨100⟩‐GaAs)] were studied using x‐ray diffraction, scanning electron microscopy, x‐ray photoelectron spectroscopy, and mass and energy dispersive recoil spectrometry. Samples were vacuum furnace annealed for time periods between 1 and 8 h at temperatures ranging from 300 to 700 °C. It was found that a CoSi2layer formed without observable reaction with the substrate at 500 °C and above. The excess Si (Si/Co atomic ratio of 2.41) remained near the surface as elemental Si and as SiO2for the 500 and 600 °C annealings. For the 700 °C annealing the excess near‐surface Si was not observed. ©1995 American Institute of Physics.