Abstract Pressure-driven flow of water and diffusion of macromolecules were studied with track-etched mica membranes to which a charged polymer, poly(styrene sulfonate) (PSS), was adsorbed. The macromolecules were dextrans of different molecular weights. The hydrodynamic experiments exhibited shear-thinning behavior; the equivalent hydrodynamic thickness of the polymer layer decreased with increasing flow rate of solvent (water plus electrolyte) through the pores. The polymer effect on molecular diffusion depended on the size of the diffusing macromolecule, with larger molecules experiencing a greater hindrance. The equivalent hydrodynamic thickness of the adsorbed polymer layer was greater than the diffusional thickness for all but the largest dextrans (molecular weight = 70,000). Both the hydrodynamic and diffusional thicknesses increased when the electrolyte (KCl) concentration was lowered, a result consistent with the notion that the adsorbed polymer layer expands at lower electrolyte concentrations due to less screening of the repulsion between charged segments of the polymer chain. The effects of electrolyte concentration were reversible unless the concentration was increased above a factor of two times the value at which the polymer adsorbed, in which case the original hydrodynamic thickness was not recovered when the electrolyte concentration was reduced.