Film growth conditions were selected to enable the extent of migration of fluoride ions in anodic oxides on tantalum to be determined precisely. By using a preformed film formed in sodium orthosilicate electrolyte in which the incorporated silicon species are immobile, a marker is generated to which mobilities may be referenced. Thus, by reanodizing the preformed film in fluoride electrolyte, secondary ion mass spectroscopy depth profiling allows precise determination of the extent of migration of the inwardly mobile fluoride ions. Fluoride ions migrate inward at a rate 1.85 times faster than that of O2- ions, leading to the development of a fluoride enriched region of TaF5 between the tantalum substrate and the outer anodic tantala oxide. From this base, consideration is given to the varied processes involved in ionic transport in amorphous anodic oxides and the requirements for enhanced understanding.