Zeta potential itself has become a very important parameter for the characterization of all kinds of filter media. To those actually involved in zeta potential measurement it is obvious to not compare quantitative numbers received by different measurement principles, since the location of the shear plane might differ between measurement principles, and measurement devices even. Additional to this well known issue, it is not always possible to condone measurements within the boundary conditions of the theory behind zeta potential measurement. The sample itself might influence measurement, for example by swelling. If one wants to determine the zeta potential of filter media using the streaming potential method, lie will find himself confronted with the surface structure of the media, and its influence on flow within the measurement zone. This work is dealing with the influence Of Surface roughness on the flow field within a measurement cell used for streaming potential measurement and its effect on the determined values of zeta potential for polyester plates and polyester multifilament fabrics.