The anodic films formed at 1 mA cm −2 on tantalum in concentrated H 3PO 4 (85%) and H 2SO 4 (95%) solutions at 25°C have been examined directly in the transmission electron microscope employing ultramicrotomed sections. In the former electrolyte, duplex films comprising an outer PO 4 [3]− -doped layer, constituting about 70% of the film thickness, and an inner relatively pure Ta 2O 5 layer are developed, contrasting with the uniformly SO 4 [2]− -doped films developed during anodizing in concentrated H 2SO 4. The doped layers contain high concentrations of PO 4 [3]− and SO 4 [2]− ions, with compositions represented by Ta 2O 5(1−x)(PO 4) 10×\\3 and Ta 2O 5(1−x)(SO 4) 5x, where x = 0.27 and 0.30 respectively. The increased incorporation of electrolyte species relative to films formed in dilute electrolytes significantly enhances the resistivity of the film material with an associated reduction in the dielectric constant.