Interactions of Cu with PtSi and TiSi2 films grown on Si (100) substrates have been studied in the temperature range of 250-500°C with Rutherfold backscattering spectrometry, Auger electron spectrometry, and x-ray diffraction. Upon annealing Cu migrates across the intermediate PtSi or TiSi2 layers and reacts with Si substrates to form Cu silicides. Unlike what were reported in the literature, no indications of the reaction of Cu with PtSi or TiSi2 have been observed. Both PtSi and TiSi2 retain their original thicknesses and compositions during Cu penetration. The structural integrity of the metal suicides in contact with Cu films is attributed to low formation energies of Cu-silicides. A 50-nm thick amorphous CoMo barrier impedes Cu diffusion up to 550°C, but fails as a diffusion barrier at higher temperatures due to CoMo-Si reaction.