A study of the electrical properties of compact scales growing on niobium has revealed several new properties of the oxidation reaction. The growth of the oxide follows two parabolic rates, with the latter approximately four times the former. Both the rate constants and the time of transition between the rates are sensitive to oxygen pressure, but it is the latter property that largely explains the extreme sensitivity of the oxidation reaction to oxygen pressure reported previously. Concomitant with the change to the faster oxidation rate is the development of a voltage across the scale (metal negative). The voltage has a value of 0.3 V at 1 atm and decreases approximately linearly with the logarithm of pressure to 0 V at 10−3 atm. The oxidation rate is essentially insensitive to induced changes in the voltage. These properties of the oxidation reaction cannot be explained by the usual formalization of point defect theory.