The influence of oxygen partial pressure and misorientation on the chemistry and bonding at interfaces between Cu and single crystalline a-Al 2 O 3 was studied by analytical electron microscopy with nanoscale resolution. The interfaces were prepared by the sessile drop technique, i.e. by melting pure Cu (99.99%) in a furnace onto a-Al 2 O 3 plates. We investigated the chemistry and bonding at the interfaces for two orientations of the single crystalline a-Al 2 O 3 substrates with two different surface planes, the basal (B-) plane, (0001), a-Al 2 O 3 B , and the rhombohedral (R-) plane, (101 ¯2), a-Al 2 O 3 R . The oxygen partial pressure (p Ox ) in the furnace was varied between 10 Ϫ4 and 10 Ϫ15 bar. This was controlled using a solid state electrolyte cell instrument. Spatially resolved (SR) electron energy-loss spectroscopy (EELS) in a dedicated scanning transmission electron microscope (STEM) was applied to characterize the chemistry and the electronic structure at the interfaces. At low oxygen partial pressure (p Ox Ͻ 3 × 10 Ϫ11 bar), all interfaces fractured during transmission electron microscopy (TEM) specimen preparation, independently of the a-Al 2 O 3 surfaces (B-or R-plane). At higher oxygen partial pressures p Ox 1:5 × 10 Ϫ10 …7:8 × 10 Ϫ4 bar the successful TEM specimen preparation indicates stronger bonding across the interfaces. No changes in the electronic structure at the Cu/a -Al 2 O 3 B interfaces as compared to the bulk were found by spatially resolved EELS while the electronic structure at the Cu/a -Al 2 O 3 R interfaces changed strongly. These differences are attributed to the reconstruction of the a-Al 2 O 3 B plane being neutral compared to the relaxation of the a-Al 2 O 3 R -plane which is charged and can therefore form ionic bonds with the Cu.