The growth of barrier-type anodic films on amorphous Al-Mo alloys, containing 16 to 40 atom percent Mo, in berate electolyte was investigated by analytical transmission electron microscopy, Rutherford backscattering spectroscopy, secondary ion mass spectrometry, and extended x-ray absorption spectroscopy. During growth at 1 and 5 mA cm(-2), two-layered amorphous oxide films formed at high efficiency by outward migration of cations and inward migration of oxygen ions; the outer layer is composed of units of alumina contaminated by boron species derived from the electrolyte and the inner layer contains units of both Al2O3 and MoO3 distributed uniformly at the resolution of the analyses. At 0.1 mA cm(-2) the films form at reduced faradaic efficiency, which results in a decreased thickness of the outer alumina layer. The two-layered films develop as a consequence of faster migration of Al3+ ions than Mo6+ ions within the inner layers of the films. New film material forms at the alloy/film and film/electrolyte interfaces and at the interface between the inner and outer layers. At the alloy/film interface, a thin layer of alloy is highly enriched in molybdenum as a consequence of anodic oxidation. Due to the mechanism of film growth, a high concentration of boron accumulates in a sublayer of film material just above the inner/outer layer interface.