To control in situ oxygen fugacity in a sample at high temperature and high pressure, the oxygen pump that is usually used at ambient pressure was introduced into a high-pressure system in the present work. Inside the sample assembly, an oxygen buffer was used as an oxygen reservoir. By means of our high-pressure oxygen pump, oxygen could be pumped from the sample into the oxygen reservoir or from the reservoir into the sample at high temperature and high pressure. Meanwhile, the oxygen fugacity in the sample could be monitored in situ by inserting an oxygen sensor inside the sample assembly. Our experiments using Ni-O system show that the oxygen fugacity in the sample at high temperature and high pressure can be conveniently controlled and monitored in situ independently of temperature and pressure. ¿ 1998 American Geophysical Union