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Titanium diboride is widely accepted to be completely wet by liquid aluminum, yet few published wetting studies demonstrate this behavior, and reported contact angles vary widely. Sessile drop substrates from four different sources were selected and their microstructures and chemistries characterized. The results of sessile drop experiments using four techniques to modify oxide film behavior were compared. The Al-TiB2 interfaces were examined in metallographic sections or after chemical removal of the Al drop. Al wets a material containing 5.5 wt% Ni in vacuum experiments before the hold temperature of 1025° C is reached. The other TiB2 substrates are completely wet by Al at 1025° C, but only after prolonged holds under vacuum. Elimination of boron oxide from the TiB2 surface leads to a spreading condition. The role of the substrate microstructure (porosity, grain size, roughness, and carbon content) in altering the wetting kinetics is discussed.