Alumina-niobium interfaces formed by liquid-film-assisted joining with copper/niobium/copper interlayers exhibited microstructures that depend on the nature of the alumina components. Characterization of these interfaces in the transmission electron microscope provided insight on the relationship between interfacial microstructure and fracture performance. Interfaces between sapphire and niobium and those between high-purity (99.9%) polycrystalline alumina and niobium were free of secondary phases. However, niobium silicides were found at interfaces between lower-purity (99.5%) alumina and niobium, identified by electron diffraction analysis as the body-centered tetragonal alpha-Nb5Si3 phase. Spatially resolved compositional analysis was conducted on silicide particles at and away from the interface.