The electropotentials set up across the scales of halides or oxides growing on metal surfaces at elevated temperatures in the corresponding atmospheres depend on the free energy change in the scale forming reactions and on the values of the ionic transference number of the scale substance. The values of the electropotentials can be obtained experimentally by means of an electrode probe; the transference numbers can then be calculated. The method was tested on silver and lead bromide and chloride and on cuprous iodide and oxide scales, and values in agreement with those found by other methods were obtained. The electrode probe method proved to be a sensitive tool responding to all factors which influence the transference numbers. The oxides on iron, stainless steel, nickel, aluminum, zinc, molybdenum and tungsten were investigated and found to be predominately electronic conductors; there is some doubt in the case of tungsten. It was shown that the Pilling‐Bedworth criterion is inadequate in the case of anionically conducting scales where the zone of growth is at the scale‐metal interface. The short‐circuiting of the scale surface and metal by means of an inert electronic conductor was found to accelerate the rate of scale growth (according to expectation). The apparent increase in the ionic conductivity of the growing cuprous oxide observed during the increase of oxygen pressure was explained by employing the concepts of the disorder theory of non‐metallic conductors.