Solid‐state reactions in mixtures of nanometer‐sized Cu and Zr as well as Ni and Zr crystallites—produced by inert‐gas condensation followed byinsitucompaction—have been investigated by x‐ray diffraction and thermal analysis. Nanocrystalline particle mixtures and multilayered samples showed qualitatively the same behavior: A solid‐state amorphization reaction occurs in NixZr1−xsamples but not in CuxZr1−x. The results are discussed in the framework of a heterogeneous nucleation and growth process for the amorphous phase. Comparison with results from ball‐milling experiments and mechanically prepared multilayers reveals the significance of additional external driving forces. ©1995 American Institute of Physics.