The evolution of surface band bending and surface photovoltage was monitoredinsituby photoreflectance spectroscopy during activation of the surface of epitaxial GaAs to the state of negative electron affinity by successive deposition of cesium and oxygen in a standard ‘‘yo‐yo’’ technique. Considerable variations of the band bending (by approximately 0.3 eV) and surface photovoltage (by three orders of magnitude) were observed. It was found that the maximum of photoemission quantum yield corresponded to unexpectedly small value of the band bending &Jgr;s=0.3 eV, as compared to widely accepted value of approximately 0.5 eV. ©1995 American Institute of Physics.