Abstract The anodizing behaviour of sputtering-deposited Al–Nb alloys, containing 21, 31 and 44 at.% niobium, has been examined in 0.1 M ammonium pentaborate electrolyte with interest in the composition and the dielectric properties of the anodic oxides. RBS and TEM revealed amorphous oxides, containing units of Nb 2O 5 and Al 2O 3 in proportion to the alloy composition. Xenon marker experiments indicated their growth through migration of the Nb 5+, Al 3+ and O 2− species, with cation transport numbers, in the range 0.31–0.35, and formation ratios, in the range 1.35–1.64 nm V −1, intermediate between those of anodic alumina and anodic niobia. Al 3+ ions migrate slightly faster than Nb 5+ ions, promoting a thin alumina layer at the film surface, although this layer is penetrated by fingers of the underlying niobium-containing oxide of relatively reduced ionic resistivity. The incorporation of units of Nb 2O 5 into anodic alumina increases the dielectric constant from about 9 to the range 11–22 for the investigated alloys.