This paper reports detailed measurements of electrical conductivity &sgr; and thermoelectric effect S in the mineral olivine and in synthetic forsterite as functions of temperature in the range from 1000¿ to 1500 ¿C and oxygen partial pressure in the range from 10-10 to 104 Pa. The two most striking observations are strong conductivity anisotropy in forsterite and a sign change in S in olivine at 1390 ¿C. These results are interpreted to show that both materials have mixed ionic and extrinsic electronic conduction under these conditions. On the basis of these interpretations, we infer that forsterite conductivity is dominated by electronic conduction in the a and b directions and probably by movement involving magnesium vacancies in the c direction, where far higher PO2-independent conductivity is observed. Olivine appears to show mixed conduction under all the circumstances observed; at low temperature, electron holes dominate but are superseded by magnesium vacancies at high temperatures. ¿ American Geophysical Union 1989