Epitaxial GaAs films have been grown by molecular beam epitaxy (MBE) on Si‐on‐insulator (SOI) formed by ion beam synthesis. Rutherford backscattering and channeling, x‐ray double‐crystal diffraction, and infrared reflection measurements have been used to characterize the epitaxial GaAs films. Experimental results show that the crystal quality of the GaAs films improves markedly towards the GaAs surface for thicker films where the minimum channeling yield drops to 10%. Infrared reflection spectra prove that crystalline GaAs films have been deposited on the SOI structures. Refractive index profiles of the GaAs films on SOI structures can be obtained by computer fitting the interference spectra. The results indicate that the crystal quality of these GaAs films is comparable to similar GaAs films deposited directly on Si by MBE.