Accurate knowledge of the shifts in valence‐ and conduction‐band edges due to heavy doping effects is crucial in modeling GaAs device structures that utilize heavily doped layers. X‐ray photoemission spectroscopy was used to deduce the shift in the valence‐band‐edge induced by carbon (ptype) doping to a carrier density of 1×1020cm−3based on a determination of the bulk binding energy of the Ga and As core levels in this material. Analysis of the data indicates that the shift of the valence‐band maximum into the gap and the penetration of the Fermi level into the valence bands exactly compensate at this degenerate carrier concentration, to give &Dgr;Ev=0.12±0.05 eV.