Cerium conversion treatment was applied on magnesium coatings in a solution containing CeCl3.The composition of the cerium conversion layer on magnesium was investigated using X-ray photoelectron spectroscopy (XPS).It was revealed that the composition predominately consisted of a three-valent state cerium compound with a relative abundance around 90％.Some of Ce3+ was oxidized to Ce4+.Electrochemical impedance spectroscopy results revealed that the total impedance of cerium-treated samples increased for a short immersion time in NaCl solution before coating deteriorating.This deterioration was due to the coating porosity increasing and the breakdown of the conversion layer.