The electroosmotic coefficient, L12, was measured for a series of bentonite specimens with different water and salt content, the salts being CaCl2, BaCl2 and MgCl2. A simple relation between the electric conductivity of the clay, κ*, and the electric conductivity of the solution in equilibrium with the clay, κ, was obtained. It is not, however, a linear relation as e.g. an Archie law dependance. When the conductivity of the equilibrium solution is less than 0.4 S/m the clay conductivity exceeds the solution conductivity. The electroosmotic coefficient of the clay specimens is found to be similar in magnitude to that of filter cakes of activated sludge. Data from literature shows that in general materials with large pores (1 µm) have lower L12 and κ* than materials with small pores. The sludge filter cakes are thus found to resemble clay and polymer membranes rather than particle plugs.