The homogeneous coarse-grained (CG) Cu-Ni alloys with nickel concentrations of 9, 26, 42, and 77 wt% were produced from as-cast ingots by homogenization at 850 A degrees C followed by quenching. The subsequent high-pressure torsion (5 torsions at 5 GPa) leads to the grain refinement (grain size about 100 nm) and to the decomposition of the supersaturated solid solution in the alloys containing 42 and 77 wt% Ni. The lattice spacing of the fine Cu-rich regions in the Cu-77 wt% Ni alloy was measured by the X-ray diffraction (XRD). They contain 28 +/- A 5 wt% Ni. The amount of the fine Ni-rich ferromagnetic regions in the paramagnetic Cu-42 wt% Ni alloy was estimated by comparing its magnetization with that of fully ferromagnetic Cu-77 wt% Ni alloy. According to the lever rule, these Ni-rich ferromagnetic regions contain about 88 wt% Ni. It means that the high-pressure torsion of the supersaturated Cu-Ni solid solutions produces phases which correspond to the equilibrium solubility limit at 200 +/- A 40 A degrees C (Cu-77 wt% Ni alloy) and 270 +/- A 20 A degrees C (Cu-42 wt% Ni alloy). To explain this phenomenon, the concept of the effective temperature proposed by Martin (Phys Rev B 30:1424, 1984) for the irradiation-driven decomposition of supersaturated solid solutions was employed. It follows from this concept that the deformation-driven decomposition of supersaturated Cu-Ni solid solutions proceeds at the mean effective temperature T (eff) = 235 +/- A 30 A degrees C. The elevated effective temperature for the high-pressure torsion-driven decomposition of a supersaturated solid solution has been observed for the first time. Previously, only the T (eff) equal to the room temperature was observed in the Al-Zn alloys.