Low-temperature scanning tunneling microscopy and spectroscopy have been employed to analyze the local electronic structure of the (111) surface of a ceria thin film grown on Ru(0001). On pristine, defect-free oxide terraces, the empty 4f states of Ce⁴⁺ ions appear as the only spectral feature inside the 6 eV oxide band gap. In contrast, occupied states are detected between −1.0 and −1.5 eV below E_Fermi in conductance spectra of different point and line defects, such as surface oxygen vacancies, grain boundaries and step edges. They are assigned to partially filled 4f states localized at the Ce³⁺ ions. The presence of excess electrons indicates the oxygen-deficient nature of the direct oxide environment. The f state spectroscopy with the STM allows us to probe the spatial distribution of Ce³⁺ ions in the ceria surface, providing unique insight into the local reduction state of this chemically important material system.