The wetting behavior of NiO single crystal by liquid aluminum has been studied by the sessile drop method under vacuum at 973-1273 K for 2 h. Optical microscopy, SEM, EDS and X-ray analysis were applied to characterize the structure and chemistry of solidified cross-sectioned sessile drop couples. Under tested conditions, molten Al wets and reacts with NiO to form Al2O3 and Ni. This leads to alloying of the initially pure Al drop with Ni to the hypereutectic composition and to the formation of a thick reaction product region inside the NiO substrate, whose structure presents interpenetrated networks of fine alumina precipitates and an Al-Ni matrix. After solidification the Al-Ni matrix and the drop have the same phase composition, which is in agreement with Al-Ni phase diagram, showing the formation of Al3Ni at T < 1128 K and Al3Ni2 at 1128 K < T < 1406 K. The strong reactivity of Al/NiO couples, accompanied with the drop deformation, fragmentation of the reaction product region and development of a crater under the drop, contributes to the perturbation of the triple line and to the formation of apparent contact angles at 1073-1273 K. This leads to unusual changes of measured contact angles with temperature, decreasing from 840 at 973 K to 360 at 1073 K. and then increasing to 750 at 1273 K, while structural analysis suggests complete wetting at 1073 K. (C) 2005 Springer Science + Business Media, Inc.