Extensive plastic deformation of metallic mixtures (mechanical alloying) leads to the synthesis of a variety of alloy phases with nanometric grain sizes. Good control of the results of alloying requires to understand the factors which influence the reaction path during the process. This paper reports the information obtained for Cu-Zn, Al-Ni, Ni-Zr and Cu-Fe from the microstructural characterization of samples prepared by ball milling and by controlled compression. It has been found that both the driving force for the solid state reactions and the effective interdiffusion coefficients are markedly affected by the plastic deformation and that an important role may be played by the presence of reactive foreign atoms.