We report a photoreflectance study of the effects of Ar+ sputtering and thermal annealing on the Fermi level (VF) on (001) n‐ and p‐type GaAs with large, uniform electric fields. The measurements were performed in situ in an ultrahigh vacuum (UHV) chamber. The effect of the sputtering was to move VF from midgap to near the conduction band for both types of material. Subsequent UHV annealing (350 °C) and air exposure restored VF its original midgap value. The implication of these observations for various models of Schottky barrier formation will be discussed. Our work also demonstrates the need to simultaneously measure both n‐ and p‐type material in order to obtain unambiguous results.