Electroviscous effects and streaming potentials were measured for various track-etched ultrafiltration and microfiltration membranes at a number of salt concentrations, using both LiCl and KCl electrolytes. A calculation program was developed that calculates zeta potentials from this experimental data, using electrokinetic flow theory and a numerical solution of the Poisson-Boltzmann equation. It was demonstrated, both experimentally and by calculation, that the electroviscous effect for negatively charged membranes is larger when employing LiCl solutions than when employing KCl solutions. This effect is absent for positively charged membranes and can thus be used for determination of the sign of the membrane charge. It was found that zeta potentials calculated from electroviscosity data were al ways higher than those calculated from streaming potential data. This indicates that the electrokinetic theory underpredicts the electroviscous effect; It was also found that at low salt concentrations and small pore sizes (kappa r(p) < 1.5) it was not possible to calculate the zeta potential from streaming potential or electroviscous effect. This may be caused by the fact that some assumptions made in electrokinetic flow theory lose their validity under these conditions.