The Co/Ti bilayer silicidation technique is widely known as a method to obtain an epitaxial CoSi2. The Co/Hf bilayer also can be used for this purpose. During the silicidation the metal and the spacer SiO2 can react. Any residue of this reaction can degrade device performance by compromising the oxide integrity or by producing pattern bridging. In this paper the reaction of the Co/Hf bilayer with SiO2, as well as that of the Co/Ti bilayer with SiO2, during the silicidation annealing are reported. The collapse of the upper side of the SiO2 substrate occurred at 800 degrees C in the Co/Ti/SiO2 sample and at 700 degrees C in the Co/Hf/SiO2 sample. The sheet resistance of both the samples increased rapidly in the temperature range above 500 degrees C. The rapid increase in the sheet resistance of Co/metal/SiO2 may be owing to the reaction between the metal layer and the SiO2 substrate, and the agglomeration of the Co layer on SiO2. Considering the temperature to get epitaxial CoSi2, along with the temperature from which the SiO2 substrate starts collapsing, we may conclude that optimum silicidation annealing temperatures for Co/Ti/Si and Co/Hf/Si are 700 and 600 degrees C, respectively.