A study of the formation of In- and Au-GaAs(100) interfaces is reported. The metal overlayers are deposited in ultrahigh vacuum on room-temperature (RT) and low-temperature (LT) (100)GaAs grown by molecular-beam epitaxy, following the evaporation of a protective As capping layer from the semiconductor surface. High-resolution photoemission spectroscopy and low-energy electron diffraction are used to characterize these interfaces. In forms a two-dimensional interface layer plus clusters at RT and LT, with reduced clustering at LT. The top substrate layers are only slightly altered by In. The Au-induced perturbation is more significant, as reflected by Ga outdiffusion even at LT. Different Ga core-level components are found at RT and LT, attesting to different levels of segregation as a function of temperature. The GaAs(100) band bending is studied as a function of metal coverage and deposition temperature. Kelvin-probe measurements, coupled with synchrotron-radiation photoemission, are performed to evaluate the synchrotron-light-induced surface photovoltage. The Fermi level is found to be pinned at 0.4 and 0.6 eV above the valence-band maximum for Au and In, respectively, in good agreement with the positions obtained on the cleaved (110) GaAs surface. Evidence of correlation between pinning and overlayer metallization is found for both interfaces.