In order to get a clear picture for describing the growth process of the oxide film formed on magnesium alloy AZ91D under plasma electrolytic oxidation (PEO) in alkaline silicate solution, the characteristics of PEO films formed at different reaction stages were systemically investigated. The results of morphologies, compositions and electronic properties indicated that the PEO films had a different growth behavior as the PEO treatment proceeding. At the initial stage (before the occurrence of sparking), the growth rate of PEO films was low, the elements (O, Mg, Al and Si) contents were varied obviously and the donor concentration in the film was kept at a high level. After sparking occurred, the PEO films showed a higher growth rate due to the high transfer rate of ions and electrons and the existence of plasma reactions; simultaneously, the films formed on alpha- and beta-phase exhibited different growth rate. With treated time increased, the thickness of PEO films and transfer resistance to ions and electrons were also increased; thereby, the growth rate of the PEO films was decreased gently. (c) 2007 Elsevier Ltd. All rights reserved.