In brine-saturated porous media the existence of a space-charge layer at the solid/liquid interface leads to a coupling between fluid and electric currents. This coupling is seen as a streaming potential (STP) across a porous plug when the fluid flows through it, or conversely, as electroosmosis (ELO) of the fluid when an electric field is applied. The magnitude of these electrokinetic effects depends on the thickness of the space-charge layer relative to the pore size, which in turn depends upon the salinity of the brine. From electrokinetic measurements one can obtain an effective hydraulic radius R eff and an effective zeta-potential at the slipplane ζ eff . We have developed a high-sensitivity AC technique that can detect these small coupling effects, and have measured them in a suite of natural and artificial rock samples. We find that R eff and ζ eff depend on salinity in ways which may be attributed to the roughness of the pore surfaces.