The operational characteristics of the oxygen pump using yttria-stabilized-zirconia electrolyte were investigated in detail. Using a closed chamber with one oxygen pump and one oxygen sensor, we show that there are lower limits of oxygen partial pressure which can be attained inside the closed chamber. The first limit corresponds to the diffusion limiting current of the pump cathode. Beyond this limit, forcing the pumping action increases the pump cathode polarization without further decreasing the oxygen partial pressure inside the chamber. Eventually, at high enough polarization, electronic conduction due to partial electrolysis of the electrolyte occurs. When electronic conduction exceeds a certain level, lower oxygen partial pressures can be realized. However this second limit (e.g., 10(-15) atm at 800 degrees C) is still much higher than values reported in the literature (10(-27) atm). Comparing the two methods of pumping control, constant applied voltage and constant current, shows that the former allows better control of the oxygen partial pressure.