Photoluminescence (PL) intensity onn‐type InP at room temperature is found to provide a convenient probe of the density of surface states in the upper part of the gap of InP. PL measurements are used to monitor the interaction phenomena occurring between an insulator (Al2O3, SiOx) and the InP substrate during the first stages of its deposition. Despite the use of a soft deposition technique, the insulator induces systematically interfacial defects in metal‐insulator‐InP structures. The nature of the insulator and, to a larger extent, the InP surface preparation and the substrate temperature (although kept below 150 °C) play a major role.