Studies were conducted to investigate the properties of the Ru–GaAs interface. An analysis was made of two sets of experimental findings, namely the distribution of surface states of GaAs covered by submonolayers of ruthenium, as well as of electrical measurements on Ru–GaAs contacts. It was found for the present case that a deep state nearEt≊0.3 eV above valence band maximum is induced at the surface. Thus the influence of the intrinsic defects is at least compensated. Using a simple model, it was investigated to which degree surface states and their energy would govern the barrier height. It was further studied how Fermi level pinning depends on the doping concentration of the semiconductor.