The temperature dependence of saturation magnetization has been observed for an unstable FCC Fe₅₀Cu₅₀ solid-solution sample prepared by mechanical alloying. The unstable Fee Fe₅₀Cu₅₀ solid solution has a Curie temperature of 505+or-5 K. At low temperatures the saturation magnetization is well approximated by the relationship M(T)=M(0)(1-BT³2/), where M(0)=108.7 emu g^-1 and B=4.21*10^-5 K^-32/. This holds over a large temperature range (T/T_c<0.4) indicating the presence of long-wavelength spin-wave excitations and disordered atomic structure. On annealing the as-milled sample at annealing temperatures above about 500 K, decomposition of the metastable FCC Fe₅₀Cu₅₀ solid solution was observed. The magnetic moment of Fe atoms in the FCC Fe_100-xCu_x Phases remains at about 2.35 mu _B for 50<x<61, and drops rapidly to 0.544 mu _B for x=70.7. This results in the 40% reduction of the saturation magnetization at zero temperature for the sample annealed at 673 K for 1 h when compared to that for the as-milled sample. The Curie temperature of the FCC Fe_29.3Cu_70.7 solid solution falls to about 200 K.