The interphase in an epoxy-aluminum system has been revealed and characterized using scanning electron microscopy, ion etching, energy-dispersive x-ray analysis, and nanoindentation. The interphase was of irregular thickness, nominally between 2 and 6 mu m, and corresponds to a region of greater resistance to ion etching and a marked absence of the silica particles used in the epoxy adhesive. Nano-indentation tests, traversing various sections of the interphase from the aluminum to the bulk resin, showed that the interphase region had, on average, an effective elastic modulus (E/(1-v(2))) that was 13% greater than that of the bulk resin, far from the aluminum. The interphase was also approximately 4% harder than the bulk adhesive.