The interaction of oxygen (O 2) with CoGa(001) at 300 K was studied by means of high resolution electron energy loss spectroscopy (EELS), scanning tunneling microscopy (STM), low-energy electron diffraction, and Auger electron spectroscopy. For low O 2 exposures (≤ 0.3 L), oxygen is dissociatively adsorbed on the surface. In STM images these oxygen adatoms appear as defect-like depressions which are mainly located at the boundaries of the two perpendicularly oriented c(4 × 2) domains of the reconstructed clean CoGa(001) surface. The oxygen adatoms are found to be immobile at 300 K. The observed distribution of the oxygen atoms on the surface can be explained by a short-lived and mobile atomic precursor oxygen species (model of “hot adatoms”) and assuming the domain boundary defects to be reactive centers for oxygen adsorption. For O 2 exposures > 0.3 L, nuclei of amorphous gallium oxide start to form. In STM images they appear as pairs or clusters of protruding features. For an exposure ≥ 25 L, the CoGa(001) surface is entirely covered with an amorphous gallium oxide film. In EELS spectra this film is characterized by intense and broad loss structures around 400 and 690 cm −1 originating from Fuchs-Kliewer modes within the oxide film.