Electron microscopy has shown that the flaws in formed anodically on contaminated or roughened tantalum surfaces are thin spots in the film. At the thin spots, which have a diameter about equal to the film thickness, the two surfaces of the film have roughly conical indentations of considerable depth so that the minimum thickness may be less than half the film thickness elsewhere on the specimen. There is probably a gap between the metal and the film under the thin spots. Flaws, which form around a point where film growth is prevented, can be produced by carbide and oxide particles only a few hundred angstroms in diameter, or by surface roughness resulting from abrasion, chemical etching, or a crack in a pre‐existing oxide film. The formation behavior and dielectric properties of films containing many flaws differ considerably from those of flaw‐free films.