Hydraulic conductivity or permeability, a knowledge of which is vital in modelling the movement of hydrocarbons in reservoir rocks and contaminants in aquifers, is strongly related to the intergranular pore-throat size in sandstones. For many years attempts have been made to determine permeability from the electrical properties of rocks, but with little success. Here we have measured the complex electrical conductivity of Permo-Triassic sandstone samples over the frequency range 0.0001--1000 Hz and show how parameters determined from the characteristic shape of the spectra correlate closely to pore-throat size determined from mercury injection measurements.