We present a novel method to measure growth rates during homoepitaxy using reflectance anisotropy spectroscopy (RAS). We have grown Si and Zn doped as well as undoped GaAs and InP layers in MOVPE. The RAS signal at photon energies near the fundamental band gap shows an oscillatory behavior (Fabry-Perot-like oscillations) during growth whenever the doping concentration has been changed. The period of these oscillations can be directly related to the growth rate via the length of the optical path through the growing layer, while the amplitude of the oscillations correlates to the doping level contrast at the interface. We attribute these oscillations to the linear electro-optic effect (LEO) at the interface between two differently doped layers. (c) 2006 Elsevier B.V. All rights reserved.