The thermal stability and the interfacial reactions in the metallization system of Al/Ti-Ta/Si for T≤550°C were studied by x-ray diffraction, transmission electron microscopy, and Auger electron spectroscopy. The analysis of this complex system was made possible by a systematic study of its subsystems. Bilayers of Al/Ti and Al/Ta, tri-layers of Al/Ti/Ta and Al/Ta/Ti, and finally Al on alloy films of Ti-Ta were deposited on Si(100) and studied. The refractory metals interactions with Al started at much lower temperatures than those with Si. In the case of the bilayer systems, Al/Ti/Si and Al/Ta/Si, the onset of interaction with Al was at 300 and 350°C for Ti and Ta, respectively, resulting in the formation of Al3Ti and Al 3Ta. The corresponding temperatures for silicide formation were 500 and 700°C. For the tri-layer systems the Al overlayer reacted with the top refractory metal at 300-350°C, while the lower metal reacted with the Si substrate at the corresponding temperature for silicide formation. For the alloy samples, reactions at the Al/refractory alloy interface at 300-350°C resulted in a mixture of Al3Ti and Al3Ta. The majority of the Ti-Ta film, and especially its interface with Si, remained intact at annealing temperatures lower than 500°C. For the Ta-rich compositions studied (Ti20Ta80 and Ti50Ta50) at 500°C Si diffused through the Ti-Ta alloy to the outer region of the contact forming Ti and Ta disilicides, while Al penetrated deeply into the Si substrate. Most of the Ti-rich alloy, Ti80Ta20, however, remained essentially intact even after 500°C 30-min anneal. At this stage a shallow contact was obtained by the formation of a very thin silicide layer at the substrate interface, while penetration of Al to the substrate was prevented by a limited interaction of Al with Ti and Ta to form the corresponding aluminides.