The research was aimed at microstructure characterization of the reaction products formed between molten aluminium and CoO single crystal during a sessile drop wettability test performed in vacuum at 700 and 1000 degrees C for 120 min using contact heating procedure. The solidified Al/CoO couples were sectioned and used for cutting thin foils with focused ion beam. The transmission electron microscopy and energy dispersive X-ray spectroscopy were used for microstructure and local chemical analysis. The interaction of molten aluminium with CoO substrate at 700 degrees C caused the formation of a corrugated 10-40 microm thick reaction zone (RZ). It consisted of aluminium matrix and Al(2)O(3) crystallites varying in size, i.e. of approximately 0.2 microm near the Al drop/RZ interface, growing up to 1-2 microm at the RZ centre and very fine nano-crystallites near the RZ/CoO interface. The reaction of aluminium with CoO at 1000 degrees C produced much thicker RZ of approximately 280 microm characterized by layered microstructure of alternating fine crystalline Al(2)O(3) and coarser Al(13)Co(4) layers. Moreover, at the RZ/CoO interface the presence of a cobalt layer was also identified.