CoNi alloy films with compositions varying from 5 to 95 at.% Co were deposited onto silicon substrates by evaporation from electron beam sources. After annealing, the reaction products were analyzed by Rutherford backscattering spectrometry, Auger electron spectroscopy, secondary ion mass spectrometry and X-ray diffraction as a function of the annealing temperature (from 450 to 750 °C). The silicide formation is influenced by the isomorphism of the compounds Co2Si and Ni2Si, and CoSi2 and NiSi2, and the lack of isomorphism of CoSi and NiSi. It is observed that the formation of the monosilicides is accompanied by a non-uniform distribution of the metallic elements as a function of depth. The formation of the alloyed disilicides always occurs at temperatures much lower than those required to form NiSi2 from pure nickel. For most cases the alloyed disilicides form at temperatures even lower than that required for the formation of CoSi2 (pure). The position where the formation of the disilicides begins varies with the composition of the alloys. In some cases the initial formation of the disilicide occurs at the free surface and is followed by inward growth; in other cases this occurs inside the complex monosilicide layer. Such effects illustrate quite dramatically the importance of nucleation phenomena in the formation of these disilicides. Nucleation of the disilicide phase at the interface between CoSi and NiSi is attributed to the contribution of the entropy of mixing to the driving force for the transformation, clearly illustrating the importance of entropy in some classes of solid state reactions. © 1985.