Poly(ethylene terephthalate) (PET) track-etched membranes with av. pore diams. of 692 and 1629 nm were functionalized using the monomer N-isopropylacrylamide (NIPAAm) and a photoinitiated \"grafting-from\" approach in which a surface-selective reaction was most efficiently achieved by combinations of the unmodified PET surface with benzophenone and, alternatively, of an aminated PET surface with benzophenone carboxylic acid. Consistent estns. of the pore diams. of the base PET membranes and of the effective grafted polyNIPAAm layer thicknesses on the PET pore walls were possible only on the basis of the permeabilities measured with aq. solns. of higher ionic strength (e.g., 0.1 M NaCl). However, the permeabilities measured with ultrapure water indicated that the \"electroviscous effect\" was significant for both base membranes. The influences of membrane pore diam., surface charge, and soln. ionic strength could be interpreted in the framework of the space-charge model. Functionalized membranes with collapsed grafted polymer hydrogel layer thicknesses of a few nanometers exhibited almost zero values of the zeta potential estd. from the trans-membrane streaming potential measurements. This was caused by a \"hydrodynamic screening\" of surface charge by the neutral hydrogel. Pronounced changes in permeability as a function of temp. were measured for PET membranes with grafted polyNIPAAm layers, and the effective layer thickness in the swollen state-here up to .apprx.300 nm-correlated well with the degree of functionalization. The subtle addnl. effects of soln. ionic strength on the hydrodynamic layer thickness at 25 DegC were different from the effects for the base PET membranes and could be explained by a variation in the degree of swelling, resembling a \"salting-out\" effect. Overall, it had been demonstrated that the functionalized capillary pore membranes are well suited for a detailed and quant. evaluation of the relationships between the synthesis, the structure, and the function of grafted stimuli-responsive polymer layers.