With the CALPHAD approach based on the regular solution model, the thermodynamic functions of the Cu-Co system can be extrapolated to compositions where the homogeneous fee solid solution is thermodynamically unstable. Up to now it has been unclear how reliable such extrapolations are in view of the driving forces for decomposition and whether the Gibbs energy is well described by the model in the unstable region. Therefore, electromotive force measurements were performed to determine directly the chemical potentials of highly supersaturated Cu-Co alloys. Under the prerequisite that time scales of the electrochemical exchange reaction are fast compared with the kinetics of decomposition, the thermodynamics of the alloys can be investigated quasi statically. Within the experimental accuracy the results for both a metastable and an unstable alloy agree well with the calculated chemical potentials. In addition, the experiments demonstrate that the electrochemical cell reaction is reversible upon small external concentration fluctuations of the investigated alloys. Possible reasons for this unexpected stability are discussed taking the kinetics of the decomposition into account.