Large changes of the refractive index (Δn≊0.25) and absorption coefficient (Δα≊16 000 cm −1) of GaAs grown by molecular beam epitaxy (MBE) at low substrate temperature (LT-GaAs) induced by annealing are reported. The refractive index difference between the LT-GaAs layer and the GaAs substrate are determined from both, the amplitude of the Fabry–Pérot oscillations and the shift of their extrema towards shorter wavelengths yielding nearly the same results. With increasing anneal temperature the excess refractive index as well as the strong absorption at photon energies below the GaAs band gap (determined by transmission measurements) disappear around 700 °C.