Four Nuclepore(R) microfiltration membranes having different pore diameters were electrokinetically characterized by streaming potential measurements. Two different modes were applied. In the permeation flow mode the testing solution is directed perpendicularly to the membrane; in the other case the solution streams tangentially across the membrane. The parameters for the investigations were pH and the concentration of the KCl solution. The aim was to investigate the influence of pore radii and surface conductivity upon the streaming potential that was simply converted into the zeta potential by using the Helmholtz-Smoluchowski equation. In the case of the tangential flow mode, a pore influence is out of question. Also the surface conductivity can be neglected in this flow mode at an electrolyte concentration of 10(-2) M, but at a 10(-3) M concentration the surface conductance contributes about 15% to the overall conductivity. Mainly the carboxylic groups of the polymeric material are responsible for surface conductivity. Under the permeation flow condition the influence of the pore dimensions upon the zeta potential is remarkable. This is due to surface conductivity and overlapping double layers. Additionally, hydrophobicity is involved too, since it increases when pore dimensions decrease depending on the amount of polyvinylpyrrolidone that is used by the manufacturer for improving the water uptake of the membranes.