This paper describes a study concerning the interaction of oxygen with clean Ge(001)2 × 1 surfaces in ultrahigh vacuum at 300 K. Surface conductance measurements, differential ellipsometry in the photon energy range of 1.5–4 eV and Auger electron spectroscopy have been used to monitor this solid-gas reaction. We observed in the joint density of states, as derived from ellipsometry, a peak at about 3.3 eV in addition to the optical transition at 1.8 eV reported earlier by Meyer. From the optical spectrum we show that the clean Ge(001) surface has at least three distinct surface states. Our results are essentially consistent with recent STM measurements and theoretical predictions. Moreover, we show that the distinct feature at 2.6 eV below the Fermi level as found with STM can be attributed to a surface state.