Electrical conductivity of orthopyroxene, clinopyroxene, and ilmenite+garnet with the same starting material San Carlos orthopyroxene (Mg0.92Fe0.08)SiO3 containing 2.89% Al2O3 by weight were measured within their respective stability fields in a multi-anvil press. In the formula for conductivity &sgr;=&sgr;0 exp(-ΔH/kT), pre-exponential factors &sgr;0 of orthopyroxene, clinopyroxene, and ilmenite+garnet are 5248, 1778, 2239 S/m, respectively, and the activation enthalpies are 1.80, 1.87 and 1.66 eV, respectively. These results are required for defining a laboratory-based conductivity-depth profile of the upper mantle. ¿ 1999 American Geophysical Union