Anodic films were formed on aluminum at 0.5 A dm(-2) to 50 V in a 1 mol dm(-3) triethylmethylammonium hydrogen maleate/gamma-butyrolactone solution of varying water content (10 to 1000 ppm). The resultant films were characterized by transmission electron microscopy, Auger electron spectroscopy, Fourier transform infrared spectroscopy, and x-ray absorption near edge structure analysis. The thickness, chemical composition, and structure of the resultant barrier-type films were affected markedly by water content with the incorporation of electrolyte components into the anodic films enhanced by decreasing water content. The film formed below 200 ppm of water proved to be an electropolymerized organic film containing a small amount of aluminum, while the film formed above 700 ppm of water was a different aluminum oxide containing a small amount of carbon. Ln the transition region between 200 and 700 ppm of water, a laminated composite film of high carbon content in the outer layer was produced. All the films showed a new infrared peak at 1610 cm(-1), which did not change upon annealing at 300 degrees C. This peak was assigned to an Al=0 stretching vibration. The permittivity of the films was lowered by incorporation of organic substances into the film material.