The differential thermal analysis (DTA) is utilized to determine the phase boundary and phase equilibria of Co-Cu alloy having a miscibility gap. Regions for various phase i.e., spinodal and coherent binodal, peritectic, magnetic and the transformation from the two phases to phase transformations are distinctly determined for the Co-13 at%Cu alloy. The obtained results might indicate the continuity of the decomposition kinetics at the boundary between instable and the metastable regions but an activation barrier is needed. The activation energy for the magnetic transformation is determined to be 182.2 ± 2.1 kJ mol-1.