The structure of the barrier-type anodic oxide formed on preannealed Al-1 weight percent (w/o) Si-0.5 w/o Cu thin film in a tartaric acid electrolyte was investigated. The oxide film is basically an amorphous layer with a thin dispersed gamma’-Al2O3 crystalline layer interposed in the center. Pores are observed to be associated with the gamma’-Al2O3 layer. Silicon nodules and Al2Cu particles originally present in the Al-1 w/o Si-0.5 w/o Cu film behave differently during anodization. Silicon nodules are oxidized to various degree during anodizing. The silica formed in the Si nodules is amorphous and somewhat porous possibly due to oxygen evolution associated with the Si anodization. A dark rim was found to surround each nodule in the anodic oxide film. This rim is shown to be thicker amorphous Al2O3 material, and its origin is attributed to the faster oxidation rate in the vicinity of Si nodules. Al2Cu precipitates are oxidized to form Al2O3 at about the same rate as the surrounding Al matrix. Copper is rejected by Al2O3 and accumulates at the Al2O3/Al interface.