A new technique to control oxygen fugacity in situ under high-temperature and high-pressure conditions has been developed. In this method a special sample assembly constructed of a reference buffer, yttrium-stabilized zirconia (YSZ) disks, a sample, and an oxygen reservoir was used. Through a driving voltage ϵ0 between the sample and the oxygen reservoir, oxygen was moved from the reservoir into the sample, or from the sample into the reservoir. The rate of oxygen transport was adjusted by regulating ϵ0 and the thickness of the YSZ disk d0. The oxygen fugacity in the sample and its variation during the process were monitored in situ by measuring the voltage between the sample and reference buffer. In this way the oxygen fugacity in the sample was controlled independently of temperature, pressure, and composition of the sample. We made experiments at 773--994 K and 1.0--4.0 GPa in the systems Cu-O, C-O, and Ni-O, and the results show that the technique is successful. ¿ 1999 American Geophysical Union