Ion beam analysis and electron microscopy have been employed to investigate the formation of anodic films on aluminum in a borax electrolyte under potentiostatic conditions. The results indicate the initial growth of a barrier film, containing boron species, followed by growth of boron-free alumina, containing pores. The efficiency of porous film formation is ̃45%, owing to the loss of Al3+ species to the electrolyte. 18O tracer is retained in the film as pores develop, which is suggested to be related to the inward transport of the O2- ions in the growing alumina. In contrast, boron species are lost from the film, which is attributed to the field-assisted dissolution of the alumina as pores develop in the boron-containing region.