The cathodic dark current at n‐type Nb‐doped and electrodes in , Fe+3, or aqueous solution was measured as a function of applied voltage, concentration of oxidizing agent, and pH. Two distinct current regimes were observed, differing by their Tafel slopes and by their concentration dependence. The most marked feature of the results was the saturation of the current as a function of concentration in the high current‐density region. The experimental data were interpreted on the basis of a model involving electron transfer through surface states.