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High temperature differential scanning calorimetry (DSC) is applied to undercool and crystallize melts of a Cu 70Co30 alloy into the metastable miscibility gap. The kinetic prefactor Γ and the activation energy ΔG* of the nucleation rate are determined based on the statistical analysis within classical nucleation theory. The value of Γ reaches 2.64 (0.21) × 1037 m−3 s−1, which is close to that of the value for homogenous nucleation and much larger than that of undercooled pure Co melts. The value of ΔG* is estimated to be 67 (2.5) k B T which is also higher than that of undercooled pure Co melts. The nucleation of the crystallization of the Co-rich phase is governed by homogeneous nucleation or conditions that are indistinguishable from homogeneous nucleation and the Cu-rich liquid phase effectively prevents the occurrence of heterogeneous nucleation for the nucleation of the Co-rich phase in the liquid-phase separated Cu 70Co30 alloy. The results indicate that nucleation of the crystalline phase is sensitively dependent on the metastable binodal, which modifies the nucleation boundary conditions, leading to an effective transition of the dominant nucleation mechanism that depends critically on the vicinity to the metastable miscibility gap.