The performance of a stabilized zirconia sensor and electrochemical oxygen pump combination in the isothermal control of oxygen fugacity in a nonflowing atmosphere was studied in a closed furnace system in the temperature range of 800° to 1100°C. Under certain conditions, large differences in oxygen pressure were found between sensors at the same constant temperature in close proximity to each other. This result reflected the existence of a stable steplike oxygen fugacity “front” in the furnace, separating regions which differed by several orders of magnitude in oxygen fugacity. This oxygen pressure profile in the furnace resulted from the steady‐state transport of oxygen from oxygen leakage sources to the pump. The existence, magnitude, and position of the oxygen fugacity front were found to depend on the gas‐phase composition, the relative locations of the leakage sources and the pump, the oxygen fugacity of the reference electrode, and the magnitude of the oxygen flux in the