A pretreat phosphate solution having the formulation of KMnO 4 , K 2 HPO 4 , Na 2 SiO 3 , NaF and Na 2 MoO 4 was developed to improve the strength of adhesive-bonded magnesium AZ31 joints. The phosphate coating which has the magnesium phosphate, MgO, Mg (OH) 2 , MgF 2 and minor Al 2 O 3, , Al(OH) 3 Al 0.35–0.55 Si 0.10–0.48 P 0.13–0.35 O 2.1–2.2 and, Al 0.35 Si 0.48 P 0.18 O 2.2 , and Al 0.52 P 0.48 O 2.2 was formed on the surface of the magnesium AZ31 after treatment. The additives Na 2 MoO 4 in a phosphate-permanganate solution mainly act as an accelerator to speed up the coating formation. The combination of Na 2 MoO 4 and NaF improves significantly the corrosion resistance of the magnesium AZ31 and performance of adhesive bonded joints. The joint strength and durability of the bonded Mg AZ31 joints in hot-humid environmental condition are superior to that of the phosphate treated (RPT) and CrO 3 etching (CET) pretreated samples.