Surface photovoltage spectroscopy with clean 2×1 reconstructed Si(001) surfaces revealed indirect transitions from the bulk valence band into a band of empty surface states at (0.59 ± 0.04) eV above it. Chemisorption of atomic hydrogen is known to finally produce a Si(001)− 1×1:H structure and was found to remove these surface states and to decrease the ionization energy by (0.34 ± 0.01) eV. This result proves the 2×1 reconstructed Si(001) surface to be ionic and buckled with the outward relaxed surface atoms being negatively charged, since Si–H bonds are partly polar with the hydrogen being negatively charged and thus tending to increase the ionization energy of the surface. Theoretical predictions based on a model with asymmetric dimers as building blocks of the 2×1 reconstruction are in good agreement with the experimental results. Furthermore, on the clean Si(001) surface an additional donor-type surface state at (0.29±0.04) eV above the valence band edge was found with a density of approximately 1011 cm−2. It is most probably caused by some kind of surface defect.