We show that mercury injection can be used to characterize the portions of rock pore space that dominate both the electrical conductivity and the absolute permeability. The resulting new expression for the conductivity formation factor is fundamentally different from the classical Archie's law but will appear similar to it in some circumstances. The predictions of our theory for both conductivity and permeability agree with the experimental measurements within expected errors. The results indicate that the absolute permeability and the conductivity formation factor can be predicted from mercury injection measurements with no adjustable parameters. ¿ American Geophysical Union 1986