Scanning tunneling microscopy (STM) and spectroscopy (STS) have been applied to study the surface electronic properties of n-type BaTiO3 ceramics under ultrahigh vacuum and at various oxygen partial pressures. I-V tunneling characteristics of vacuum-annealed BaTiO3 do not exhibit rectifying behavior, implying that the Fermi level is pinned at the surface. The surface band gap of BaTiO3 annealed under vacuum at 540 degrees C is equal to 1 eV. The top edge of the surface valence band is located 0.7 eV below the Fermi level. Hysteresis in the I-V characteristics has been observed at high oxygen partial pressures. Dosing of the BaTiO3 with oxygen increases the surface band gap and unpins the Fermi level. As a result, the I-V characteristics acquire rectifying features similar to those observed for BaTiO3 Schottky-type diodes. Hysteresis in the I-V spectra observed at high oxygen partial pressures is attributed to the changes of the surface potential barrier due to adsorption/desorption of oxygen modulated by the tip-sample potential difference.