The applicability of Darcy＇s law for explaining the water permeabilities of polymeric UF membranes and ceramic MF membranes was investigated at various temperatures, viscosities, salt concentrations, and transmembrane pressures. It was found that Darcy＇s law explains the permeabilities well if the viscosity is corrected for electroviscous effects, temperature, and solute concentration. For polymeric UF membranes the compaction of the membranes needs to be taken into account as well. Ceramic MF membranes do not seem compressible for TMPs up to 80 kPa. Increasing the salt concentration from 30 mu M to 0.1 M resulted in increases in water fluxes of 2% to 8% both for MF and UF membranes. This apparent permeability increase was explained by electroviscous effects: increased salt concentrations lead to lower zeta-potentials and thinner double-layers, offering less resistance to water passage. From the apparent permeability change the zeta-potentials of the membranes at pH approximate to 7 were calculated. Realistic zeta-potential values were obtained. Water flux measurements at various salt concentrations are thus a simple method to study zeta-potentials of membranes. The resistance (R-m) of the UF membranes was independent of temperature In the range 4-20 degrees C, but increased with increasing transmembrane pressure (TMP). The increase could be described by a power law Delta R-m similar to TMP0.8, typical for porous solids. The resistance of the ceramic MF membrane was independent of temperature and independent of TMP.