Electro-osmotic coefficients are determined experimentally and compared to numerical data. The numerical model assumes random packings of grains of various shapes; the prediction corresponds to a modified analytical solution. A special set up was designed to measure all relevant properties independently as functions of the porosity and electrolyte concentration. Two model clays and a compact natural clay were used. Good agreement was found between experimental and numerical results, indicating that the electro-osmotic coefficient can be predicted from conductivity and permeability.