In this work we present a comparative study of the electronic and atomic structure of the interactive Cu/Si and Cu/InP interfaces. We use surface sensitive techniques, including soft x‐ray photoemission and photon‐ and electron‐induced Auger electron spectroscopy (AES), to show the strikingly similar chemical reactions and electronic properties at both interfaces. For both Cu/Si and Cu/InP, the valence band spectra and core level intensity profiles show intermixing of the metal with the substrate atoms. Moreover, for both semiconductors, the increase in Cu coverage induces an increase of the width of the Si L2,3VV (∼92 eV) and P L2,3VV (∼120 eV) Auger peaks at the earliest stage of barrier formation followed by a 4.4 eV splitting for higher coverages. These experimental observations lead to very similar models of the chemical reaction at the interface and the formation of the intermixed region and indicate bonding between Cu and P or Si for the InP and Si interfaces. In the case of the Cu/InP interface, we observe segregation of In to the surface and the formation of metallic islands.