When the anodic films of Ta2O5or of SiO2, each having the thickness 100∼2500 Å, are heated at high temperatures in contact with MnO2layers, they exhibit rectifying actions ofp‐i‐njunction type as well as large electromotive forces in the direction of easy flow resulting from the ion flow through the films. The short‐circuit ion current increases greatly with increasing temperature, while the open‐circuit voltage increases rapidly at first with increasing temperature, reaches a maximum, and then decreases gradually at higher temperatures.The results obtained are analyzed in the light of ap‐i‐njunction structure in the film. In the steady states the observed current through the external circuit is equal to the sum of the ion current, the generation, and recombination current, and the leakage current of the junction. Theoretical expressions having an explicit form are given forIionandIf, by making the assumptions that (a) the built‐in voltage in the junction is equal to the free‐energy change in the reaction of film formation and (b)Ifis of the Sah‐Noyce‐Shockley type. Agreement between the theory and experiments is excellent. We believe that the present method of investigation could also be applied in the elucidation of the growth mechanism as well as of the electric properties of films formed on other metals or semiconductors.