The complex electrical impedance of sintered, polycrystalline compacts of San Carlos olivine (grain size 75--180 &mgr;m) has been determined over the frequency range 104 to 10-4 Hz, at temperatures between 800 and 1400 ¿C, at the fO2 of the wustite-magnetic buffer. Complex plane plots (Im Z vs. Re Z) show separate effects of grain interior and grain boundary conductivity. Grain interior transport controls the response above ~102 Hz, whereas grain boundary transport dominates between ~102 and 10-2 Hz. Overall response of the compacts is modeled with an equivalent circuit in which parallel RC circuit elements representing grain interior and grain boundary responses act in series. Bulk DC conductivity is lower than either grain interior or grain boundary conductivity alone. Inversion of magnetotelluric profiles that employ lower bulk conductivities of the grain interior-grain boundary system yield upper mantle temperature estimates 30 to 50 ¿C higher than those based on single crystal values. ¿American Geophysical Union 1990