The effect of Pt and Ge on the stability of NiSi films has been examined. The addition of a small amount of Pt (5 at%) in the Ni film increases the disilicide nucleation temperature to 900oC leading to a better stability of NiSi at high temperatures. For Ni films on Si1-xGex with x=0.29 and 0.58, no NiSi2 was found after annealing at 850°C. The increase in thermal stability of NiSi has been explained in terms of nucleation concept. Calculated ternary phase diagrams allow to understand the effect of the third element (Pt or Ge) on the driving force for nucleation. The redistribution of this element can also be explained with the ternary phase diagrams.