At some metal(M)–silicon interfaces, such as in Ti–Si and Ni–Si diffusion couples, an amorphous silicide forms at low annealing temperatures, while such a reaction does not occur at others, such as Mo–Si and Co–Si. As Co–Si and Ni–Si have similar phase diagrams, silicide crystalline structures, and lattice parameters, their differing behavior in this regard has motivated a comparative study. We have used cross-section transmission electron microscopy to investigate interfacial reactions in evaporated Ni–Si and Co–Si four-layer stacks. The structure of the Ni–Si as-deposited films show intermixed amorphous interfacial layers, similar to those observed in sputtered Ti–Si and Mo–Si multilayers. These interlayers grow on rapid thermal annealing at 250 °C for 1 s, forming an amorphous Ni–Si alloy. There are intermixed interfacial layers in the Co–Si stack as well; however, they are partially crystalline. A 250 °C anneal does not change this microstructure; however, the interlayers crystallize more fully during a 10-s anneal at 300 °C. These results are discussed, along with observations in sputtered Ti–Si and Mo–Si multilayers, to clarify the factors which determine the tendency of a M–Si system to undergo amorphization.