A simple experimental setup for the measurement of electro-osmotic flow through porous media by direct weighing is described. Potential instead of current measurement allows the determination of zeta potential, even in the presence of significant surface conduction. Flow equations valid for porous media composed of monodisperse spheres are used successfully for isotropic packed beds made with randomly shaped and sized Nylon grains. The measured zeta potentials are in agreement with those deduced from electrophoresis. Copyright 1999 Academic Press.