150 nm films of Co and Ta were electron gun codeposited onto Si (100) and Si (111) substrates. The reactions of the components of the amorphous film were studied in the temperature range 873-1373 K by X-ray diffraction and transmission electron microscopy. The first phase to crystallize from the amorphous film was Co2Ta which is stable in the temperature range 873-1073 K. CoSi appeared at 973 K and was stable up to 1073 K. The end products of the heat treatment were CoSi2 close to the Si substrate and TaSi2 as the outer layer. Complete separation of the silicides occurred, as expected from a system comprising a refractory metal and a near-noble metal on Si substrate. Reactions on Si (111) occur at a faster rate than on Si (100). These take place at higher temperatures than the formation temperatures of the respective silicides in the binary systems. No CoTaSi ternary phase was observed in the present work.