Abstract The boundary between copper (111) and 0.5M H 2SO 4 was investigated using interfacial photoemission spectroscopy. Exploiting knowledge of electron momentum restrictions a feature in the ratio of p-polarized to s-polarized photoelectron yield was correlated with the onset of transitions to Bloch-like final states within the metal at 4.1eV. This provided some energy specification in the electron energy integrated yield data. We have used our understanding of this process to monitor the quality of the interface using both clean and adsorbate (thiourea, Br −) covered Cu surfaces. The major effect of adsorption is attributed to a loss of conservation of electron momentum parallel to the surface during the emission process.