Sn-plating of aluminum alloy before and after a post-molybdate treatment is characterized by 119Sn conversion electron Mössbauer spectrometry (CEMS). CEMS results gave the direct evidence that the oxidation resistance of Sn-coated aluminum alloy is improved by the post-molybdate treatment. Depth selective CEMS showed that the coating structures consist of SnO2 on the top coating and the mixed Sn(0) and Sn(II) species in the intermediate layers. The Sn(II) oxide exists abundantly near the interface between the aluminum alloy and the Sn coating rather than beneath SnO2 layer.