We, therefore, investigated the P-rich InP(001) surface and its interaction with water using X-ray (XPS) and ultraviolet (UPS) photoemission spectroscopy as well as low energy electron diffraction (LEED). In particular, we focus on the surface Fermi level position, which is found to be pinned close to mid-gap. Ab initio thermodynamics and band structure calculations show that this is related to surface defects. Furthermore, the surface reactivity of the defective surface in contact with water is investigated by modeling the semiconductor-electrolyte contact under ultra-high vacuum (UHV) using the "frozen electrolyte" approach .