When aluminum metal was anodically oxidized in quarternary ammonium carboxylate/gamma-butyrolactone electrolytes under nearly anhydrous conditions, very different oxide films were formed : the phthalate solution gave a fibrous porous-type film where phthalate anion was slightly incorporated only on the surface, whereas maleate solution afforded a barrier-type composite aluminum oxide film which contained substantial amounts of maleate anion as an oxygen source.