Abstract: Cerium-rich conversion coatings was deposited on porous film of anodized aluminum by cathodic electrolysis in a solution containing 1g/L CeCl₃·7H₂O and 2.7g/L H₂O₂. Electrochemical and chemical reactions related with coating formation were studied using electrochemical methods such as cyclic voltammetry, φ-t  curve and j-t  curve tests. It was revealed that the formation of cerium conversion coatings involves a chemical dissolution process of anodized film and an electroreduction process of 4-valent-state cerium compound. Reduction of H₂O₂ was considered to be the major factor responsible for the occurrence of the two processes, by changing the local pH at surface of anodized film before cathodic potential reached up to hydrogen evolution potential. While the reduction of O₂ was unable to trigger the occurrence of the above processes.