Pt, Ir, Ni, Pd, and silver oxide Schottky contacts were fabricated on the Zn-polar surface of hydrothermally grown bulk ZnO. A relationship was observed between the barrier height of the contact and the free energy of formation of the “metal” oxide. This is consistent with the dominating influence of oxygen vacancies (VO) which tend to pin the ZnO Fermi level close to the VO (+2,0) defect level at approximately EC - 0.7 eV, where EC is the conduction band minimum. Valence band x-ray photoemission spectroscopy and the current - voltage characteristics of planar Schottky diodes, measured on similar Zn-polar surfaces, showed the existence of a vacuum activated surface accumulation layer. This is possibly a consequence of the observed OH termination of the Zn-polar surface. The surface accumulation layer is compensated in atmospheric conditions by the presence of acceptor-like adsorbates, such as O2 and H2O. The formation of high quality Schottky contacts to ZnO should therefore involve the reduction of near surface oxygen vacancies and the removal of H or OH from the surface.