The dealloying of rapidly solidified Mg–Cu alloys in a 0.2M NaCl aqueous solution has been investigated using electrochemical measurements. The results show that nanoporous copper can be fabricated by electrochemical dealloying of the Mg–Cu alloys in the NaCl solution. The electrochemical activities of the Mg–Cu alloys are related to both alloy composition and phase constitution. The critical potentials of biphasic Mg–Cu alloys are determined by the less noble Mg 2 Cu phase, rather than being in proportional to the contents of constitutive phases or elements. Furthermore, the dealloying of Mg 2 Cu has a promoting effect on that of the MgCu 2 phase.