The corrosion behavior of the ternary Mg(2-x)Mm(x)Ni (Mm = Ce-rich mischmetal) alloys synthesized by the ballmilling diffusion method have been investigated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization. X-ray diffraction results show that in order to maintain the Mg2Ni-phase structure, the content of Mm should be controlled to less than 0.20. EIS results show that after substitution of Mm for Mg the alloys have much lower reaction resistances than Mg2Ni. Potentiodynamic polarization results show that the addition of Mm has little effect on corrosion potential (E-corr) but largely reduces the corrosion speed (i(corr)). Energy-dispersive X-ray analysis is also used to investigate the element distribution on the alloy's surface.