Fayalite, Fe 2SiO 4, forsterite, Mg 2SiO 4 and the olivines Fe 2SiO 4-Mg 2SiO 4 with 1, 10 and 50 mole % fayalite have been synthesized. The electrical conductivity of these samples has been determined by means of a suitably modified Bridgman anvil press, and is given by σ = σ σ exp ( −E kT ). E decreases linearly with increasing pressure. The effect of solid solution of forsterite in fayalite is to increase the activation energy E. The mechanism of conduction is discussed and results are applied to the estimation of the electrical conductivity of the mantle of the Earth down to depths of 500 km.