The formation of CoSi2 and TaSi2 phases from an amorphous Co-Ta film, which was obtained by electron gun codeposition on Si(100) and Si(lll) substrates, was investigated by in situ X-ray diffraction. The first phase to crystallize from the amorphous film was Co2Ta which is stable in the temperature range of 873-1173 K. Transformation proceeds by formation of CoSi, then CoSi2 and finally the TaSi2 phase is formed. On Si(lll) substrates the TaSi2, phase is detected earlier and at a lower temperature than on Si(100), and the reverse is true regarding CoSi2 formation, i.e. it was observed at an earlier stage on Si(100). The absence of the TaSi2 phase at 1073 K on Si(100) is explained in terms of the thickness of the CoSi2 phase on Si(lll) and Si(100) respectively through which the Si atoms have to diffuse to enable TaSi2 formation. The growth of CoSi2 obeys a parabolic relation. The activation energies of CoSi2 formation are 1.97 and 1.39 eV on Si(100) and Si(lll) substrates, respectively. The data for the TaSi2 phase formation can be represented by Avrami's relation. The activation energies derived from the slopes of the Arrhenius plots for TaSi2 formation are 3.07-3.23 and 2.09 eV on Si(100) and Si(lll) substrates, respectively.