Among the epitaxial transition metal silicides CoSi2 is of special interest for applications in ULSI technology due to its high thermal stability and low electrical resistivity. The solid phase reaction of metallic bilayers with Si in an N-2 ambient is a well-known process for growing epitaxial CoSi2. However, the real function of the so-called diffusion barrier, which is arising during the annealing process, remains unclear. TEM studies revealed the intermediate, growth of a CoSi phase with grains of preferred orientation to Si. The growth of this phase raises the temperature of CoSi2 nucleation, which starts at the CoSi/Si interface. The nucleation temperature determines the quality of the growing epitaxial CoSi2, which is better for Ti as a barrier forming material than for Hf, also used successfully. Whether an intermediate oriented growth of CoSi is promoting the epitaxial quality of CoSi2 otherwise than by shifting the nucleation temperature, remains an open question.