The interface formation of Cr/NH3/GaAs(100) and its temperature dependence using synchrotron radiation photoemission and resonance photoemission spectroscopies have been investigated. We observed that at T=90 K the initial deposition of Cr stimulated nitridation of the GaAs substrate and subsequent interface reaction was characterized by Cr-induced substrate disruption. Annealing of the thus formed interface to room temperature and above caused further Cr–GaAs reaction. The metallicity of the Cr overlayer was examined using constant initial-state spectroscopy (CIS) of Cr 3d with Ei=1.7 eV below the Fermi level. The CIS spectra showed a maximum at the photon energy hν=50.0 eV followed by a second broad maximum centered at hν=56–58 eV depending on the stage of the interface formation. As the nominal Cr coverage increases from 1 to 10 Å, the valence band photoemission spectra show a shift of the Cr 3d band toward the Fermi level. At the same time, the second maximum in CIS gradually dominates over the first, suggesting the evolution of the Cr overlayer to a more metallic state. The sensitivity of the second maximum in its peak position and intensity to the density of states near the Fermi level indicates that this maximum likely results from many body electron interactions. The results show that CIS can provide valuable information about the transition of a thin overlayer to a metallic state.