Corrosion protective composite coatings were prepared on AZ31 Mg alloy using plasma electrolytic oxidation (PEO) and cerium conversion (CeC) coating, and their electrochemical corrosion properties were evaluated by potentiodynamic test and electrochemical impedance spectroscopy (EIS). The PEO coating was porous and composed of Mg2SiO4. The CeC treatment was optimized at pH = 3, which exhibited the highest impedance at low frequencies. The CeC coating was initially formed at the PEO coating/substrate interface and continuously developed filling the pores and defects of PEO coating. The CeC coating was more likely to be amorphous Ce2O3 along with CeO2. The potentiodynamic and EIS tests in 3.5 wt% NaCl solution indicated that the corrosion protection of AZ31 Mg alloy was improved by PEO/CeC composite coatings. The enhanced corrosion protection of composite coatings reflects that the pores or defects of PEO coating were effectively sealed by cerium oxides and the composite coating/substrate interface acted as a good barrier to the aggressive electrolyte. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.005303jes] All rights reserved.