Atomic hydrogen is known to stabilize the InP(001) 2×2-2D-2H (Fig. left) surface, leading to completely filled and empty surface states close to the InP valence and conduction band egdes, respectively, even though an occupied surface band is located slightly above the valence band . For that reason, the observed Fermi level pinning at mid-gap has to result from surface defects inducing electronic states within the bandgap of InP. Starting from the 2×2-2D-2H surface, we consider four types of hydrogen related defects, namely desorption and adsorption of one or two H atoms, respectively. The total probability of any of these defects can be calculated from Eq. 1, which is shown in dependence on temperature and partial H 2 pressure in Fig. . At room temperature and for a hydrogen partial pressure in the range of 10 -10 to 10 -7 Pa, a surface defect density in the order of 10 10 to 10 12 cm -2 can be expected. This is sufficient to lead to surface band bending and is in good agreement with the calculated surface charges experimentally evaluated from Eq. 5. The desorption of one hydrogen atom from the 2D-2H surface is the by far predominant defect type under these conditions, leaving one phosphorous dangling bond at the surface. It corresponds to the formation of the InP(001) 2×2-2D-1H surface shown in Fig. (right). How do the surface electronic properties change upon H desorption? The band structure and the electron density of states (DOS) for the 2D-1H surface is shown in Fig. . It reveals that the H vacancy gives rise to an in-gap surface state feature. The atom-resolved DOS demonstrates that this state is of p character and is primarily due the partially filled P dangling bonds. In comparison to the valence band spectra, the dangling bond feature is located slightly below the calculated mid-gap position, which is in good agreement to our photoemission spectra, indicating that the surface P dangling bond is the dominating defect causing the Fermi level pinning at the present P-rich InP(001) surface.