An atomic force microscope has been used to study the electrical double layer interactions between a silicon tip (with an oxidised surface) and two polymeric membranes, one microfiltration (nominally 0.1 mu m) and the other ultrafiltration (25 000 MWCO), in aqueous NaCl solutions. Force-distance curves were measured for the two membranes at four ionic strengths. The membranes were also imaged under the same conditions using electrical double layer repulsive forces of differing magnitudes - "electrical double layer mode" imaging. Image analysis was used to determine surface pore size distributions. The force-distance curves, together with numerically calculated potential profiles at the entrance to a charged pore, allow an explanation and identification of the optimum imaging conditions. The best images were obtained at high ionic strength with the tip close to the membrane surface.