The oxides at the diffusion-bonded interfaces of a pure aluminum and Al binary alloys (Al-Mg (0.6-2%), Al-Si (1%), Al-Mn (0.5%), Al-Zn (1%), Al-Cu (1%)) have been investigated by transmission electron microscopy in order to examine the effect of their morphologies on the bond strength of the joint interfaces. As the bonding temperature and Mg content were increased, the strength of the joint interfaces of the Al-Mg alloys increased to levels not less than that of the base metals, and the interfacial oxide altered from continuous amorphous films to dispersed crystalline particles of Al2MgO4, and MgO. For the pure aluminum and the alloys, other than those of the Al-Mg system, the strength of the joint interfaces was much lower than that of the base metals at all bonding temperatures employed, and the interfacial oxide remained as amorphous films. These results suggest that the amorphous oxide film prevents an increase in the strength of the diffusion-bonded interface of the Al alloy and that strengths comparable to those of the base metal can be obtained, when almost all amorphous films alter to crystalline particles. When the foils of the Al-Mg alloys were applied as an intermediate layer, the bond strength of the joints of the pure aluminum and the alloys, other than the Al-Mg alloys, was improved significantly, and the oxides at the bond interface became crystalline particles in the same way as those observed in the joints of the Al-Mg alloys. Considering that the morphological change of the interfacial oxide described above can be explained as a consequence of the reductive reaction with Mg, as reported in a previous paper, it can be concluded that the addition of an element having strong affinity to oxygen, like Mg, into the intermediate layer as well as the base metal can improve the bond strength of the joint interface by changing the morphology of the interfacial oxide from amorphous film to crystalline particle.