Potentiometric sensors of the configuration La2CuO 4|YSZ|Pt are sensitive to ppm-levels of NO, NO2 and CO gas while they are insensitive to %-level variations in oxygen concentration. The voltage response is determined by the Differential Electrode Equilibria. The equilibria attained at each electrode is a function of the catalytic and electrocatalytic activity as well as the often overlooked semiconductor character of the electrode. In fact, many metal oxides used as potentiometric sensor electrodes are semiconductors. The Fermi level of a semiconductor is very sensitive to gas adsorption and changes proportionally as the gas concentration does. The Fermi level can be directly related to the electrode potential. Therefore, adsorption-induced Fermi level changes make a contribution to the potentiometric response when semiconductors are used as electrode materials. We have fabricated, characterized, and compared potentiometric and conductometric sensors with La2CuO4 sensing electrodes to illustrate this effect. copyright The Electrochemical Society.