Experimental measurements of magnesiow¿stite electrical conductivity at Fe/Fe+Mg 0.05 to 0.2 and Fe3+/Fe3++Fe2+ 0.01 to 0.7 at high pressure and high-temperature are presented. Below 1000 K, conduction occurs by a small-polaron process of electron hopping between ferric and ferrous sites, but above 1000 K there is a change in mechanism. This high-temperature mechanism is postulated to be a large-polaron process in which holes are promoted in the oxygen valence band via the reactions: 1/2 O2=OOx+VMg″+2h* and FeFe*=FeFex+h*. The hole and its associated polarization field are free to move in the valence band until trapped by a ferrous ion. Activation energies for the low-temperature, small-polaron regime are ~0.3 eV across the range of Fe/Fe+Mg and Fe3+/Fe3++Fe2+ studied, in agreement with previous studies. The high-temperature, large-polaron activation energy decreases with increasing Fe/Fe+Mg and decreasing Fe3+/Fe3++Fe2+, ranging from 0.4 to 1.1 eV. Both regions show a small, negative activation volume (ΔVlt=-0.33(19) cm3 mol-1; ΔVht=-0.26(69) cm3 mol-1), consistent with previous high pressure studies of electronic conduction mechanisms. A compilation of the available data shows a discrepancy between measurements at low and high-temperatures, consistent with the new results presented here. At the temperature of the lower mantle, the dominant conduction mechanism in magnesiow¿stite will be the more mobile large-polaron process. This is less sensitive to iron content than small-polaron conduction at Fe/Fe+Mg<0.17 (the likely compositional range of lower mantle magnesiow¿stite) and has a different temperature dependence from the low-temperature process. ¿ 2000 American Geophysical Union