Although both the Cu/W and Cu/Ta alloy systems are known to be immiscible, the response of coevaporated amorphous films ofa‐CuxW1−x(x=0.45–0.72) anda‐CuxTa1−x(x=0.10–0.55) alloys to thermal annealing and ion irradiation is very different. Alloys ofa‐CuxTa1−xcrystallize between 600 and 800 °C, with the crystallization temperature increasing with Ta concentration, whilea‐CuxW1−xalloys crystallize below 200 °C. Likewise in Xe‐ and Ne‐ion irradiation studies,a‐Cu/Ta can still be observed after ∼13 displacements per atom (DPA), whereasa‐Cu/W alloys completely transform to solid solutions after ∼0.08 DPA. The results of this study, in combination with previous investigations of amorphous alloys of immiscible systems, show that the crystallization temperatureTcis not predicted equally well for systems with a positive or negative heat of formation. It appears that the kinetic model of crystallization only applies if the transformation proceeds through long‐range diffusion. Amorphous alloys which have a positive heat of mixing are predicted to undergo a polymorphic transformation at lower temperatures.