The formation of a ternary solid solution NiSi1-xGex, instead of a mixture of NiSi and NiGe, is found during solid-state interactions between Ni and various Si1-xGex films ranging from pure Si to pure Ge. The lattice parameters of the solid solution of orthorhombic structure increase linearly with Ge content (x) as: a=5.24+0.19x Å, b=3.25+0.16x Å, and c=5.68+0.15x Å. The specific resistivity increases from 17 μΩ cm for NiSi to 21 μΩ cm for NiSi0.71Ge0.29 and NiSi0.42Ge0.58. Although the Ge content rapidly drops from 30–60 to about 10 at. % in the solid solutions formed above 600 °C, the crystallographic structure remains unchanged and no NiSi2 [or Ni(Si,Ge)2] is found in the Si1-xGex samples even after annealing at 850 °C. Without Ge, the NiSi completely disappears at 750 °C. These results indicate a strong effect of the entropy of mixing in NiSi–NiGe on the nucleation of NiSi2. © 2002 American Institute of Physics.