The surface charge or electrical potential properties of microfiltration, ultrafiltration and nanofiltration membranes can have a very significant influence on their separation performance. Such properties are most commonly quantified in terms of zeta-potentials obtained by calculation following experimental measurement of streaming potentials. Such calculation requires numerical solution of the equations governing fluid flow and electrical-potential distribution in the pores. A method for such calculations is presented, which includes a numerical solution of the non-linear Poisson-Boltzmann equation and allows for the mobilities of anions and cations to be individually specified. By expressing the results of such calculations in terms of a factor to be applied to a classical analytical result, it is shown to be very important to use proper numerical calculations in the interpretation of electrokinetic data for membranes. Use of a classical analytical analysis to calculate ＇relative＇, ＇apparent＇, ＇equivalent＇ or ＇nominal＇ zeta-potentials is likely to lead to substantial underestimation of the true zeta-potential and possible serious error even in the interpretation of relative changes in membrane properties. The calculations needed to avoid such difficulties may be readily carried out on a PC. It is also important to account for the individual mobilities of the anions and cations in the electrolyte used for measurements.