Electrical properties for a range of commercial membranes have been determined by electroosmosis and streaming potential measurements under identical conditions, For both techniques, the pH range was 3-7 and the electrolyte used was 10(-3) M KCI at 25 degrees C, as the charges on the membrane are strongly dependent upon the pH and the ionic strength of the solution passing through the membrane, For both ultrafiltration (UF) and microfiltration (MF) membranes the absolute values of the apparent zeta potential determined from electroosmosis were generally greater than those from streaming potential measurements with a greater difference at pH less than or equal to 4. The apparent zeta potentials obtained from electroosmosis were negative for all membranes studied, even at pH below the isoelectric point where streaming potentials were positive, A substantial increase of the negative zeta potential at low pH (similar to pH 4) was interpreted in terms of the effect of the applied electric field on distribution of ions and differences in mobility of ions, particularly H+ compared to the other ions, The magnitude of the zeta potentials determined from electroosmosis exhibited an apparent difference due to change in orientation of the membrane for asymmetric UF membranes, whereas the streaming potential measurements showed less directional-effect for both UF and MF membranes.